To remove P or not to remove P, that is the question.

Phosphate: to those like me who answered their Chemistry O level multiple choice by rolling a pencil down the desk because that way they tended to get higher marks, phosphate is just another chemical. But in terms of river ecology in general and chalk stream ecology in particular, phosphate is very, very important.

That’s why I’ve spent the past few weeks adding info to the Abstraction Sensitivity Band table I published in the previous post in December. The Chalk Stream Table now includes the Water Framework Directive (WFD) status for Flow and Phosphate.

Flow is a “supporting element” and so it is adjudged either to support or not to support Good Ecological Status. In WFD lingo these are the acronyms SG or DNSG. The column for the WFD ‘element’ Phosphate (P), is classified as either High (which means High status and therefore low phosphate concentrations), Good, Moderate, Poor or Bad (Bad means a high phosphate concentration). Phosphate is a pass or fail element: if P readings are Moderate, Poor or Bad the waterbody can’t be deemed to meet Good Ecological Status. There are no absolute readings used to assess status: it is somewhat tailored to each stream. Generally however, P needs to be lower than 0.036 mg/l for the chalk stream to achieve High status.

(Note: I have indicated which Sewage works remove Phosphate as best I can with the information available. The table may include some errors in that regard. Also I have included the actual P readings from the Dorset streams but not the rest (not enough time in the day), but I will in due course. There is no one source of information on this important issue of Phosphate and sewage treatment standards, which is something I hope the new chalk stream hub we are developing will resolve.)

I’ve taken P here as something of a totem for water quality in general, although that is a massive oversimplification. Nevertheless P has a huge impact on the ecology of our chalk streams. If you want to read an authoritative explanation from the expert on this subject I refer you to Phosphorous and River Ecology by Chris Mainstone. This is my Ladybird Book simple version:

Phosphorous is the key chemical that drives nutrient enrichment of chalk streams. That enrichment has a number of deleterious effects on a river’s ecology which increase in line with increasing P enrichment. All plants need P to grow, but different plants and plants communities either thrive or conversely suffer and are out-competed at differing levels of P concentration.

Higher order and important chalk stream plants like Ranunculus thrive at very low, background natural P concentrations. The first effect of P enrichment is actually an increase in the growth-rate of the higher order plants, but with commensurate weakening in root growth – making the plants vulnerable in high flows. As P levels increase further the river’s ecology shifts towards a dominance of the higher order plants that are most tolerant of nutrient enrichment, and that leads to a reduction in the overall bio-diversity of the plant community. 

Finally, if P concentrations keep on rising, the river’s ecology will switch over to an algal-dominated plant community. Benthic algae smothers the river bed and the interstices in the gravel in which many insect species live and epiphytic algae cloaks the leaves and stems of the higher order plants, reducing their ability to photosynthesise. The prevalence of algae will also cause extreme diurnal variations in dissolved oxygen levels, with really low oxygen levels at night and in the early morning, which stresses fish and insects alike. There comes a point where, if the P is very concentrated, the river turns into an anoxic soup and nothing much survives.

P is very limited in a natural chalk stream system. But P is contained in human sewage (treated and raw) and animal slurry, as well as in agricultural fertilisers. P is used in cress farms and there is P in the food used in fish farms and in the poo from those fish. There are other diverse sources of P: our drinking water is dosed with P, for example. Consequently there is much more P in our anthropogenically impacted river systems than might be considered natural. Relatively modest increases in P can cause the ecological changes outlined above. Any reduction in P will benefit the ecology of a chalk stream, but if P levels are high, or there’s loads of it still washing around the system, you might have to reduce P by an awful lot before you start noticing the difference.

There are also complex relationships between different states of P, which might be dissolved within the water column (Soluble Reactive P) or bound to organic and inorganic particles which accrete on the river bed: P can move between these two during its journey through a river system, meaning that P locked away within the sediments on the river bed can be reanimated in high flows when the river bed is disturbed.

P gets into a chalk stream through “Point Source” and “Diffuse” pathways.

The main Point Source supply of P is through the human sewage system, but fish farms and cress farms are also Point Source suppliers: a large fish farm (40 tonnes annual production), for example, can generate as much P as a secondary Sewage Treatment Works (STW) serving 1000 people.

Diffuse Source P, on the other hand, flows in multiple pathways from the wider landscape, and particularly from farmland. The majority of Diffuse P gets to the river by surface or shallow sub-surface flow during the wet winter months, when soil is saturated. 

A clear pattern you will see in this table is the correlation between chalk streams where there are no sewage treatment works (STWs) or chalk streams where the STWs includes a tertiary P stripping phase which tend to be of Good or even High status for P.

And conversely between chalk streams which have one or several STWs which do not remove P and which tend to be of Poor, Moderate or Bad status for P.

On some rivers I have noted in the r/h column where the P readings were markedly different upstream and downstream of a STW. One stark example is the Misbourne and I will publish a revealing chart of that in the next post.

There are other clear patterns: the relatively higher status for P on the larger systems as you move in a downstream direction. This is because the Urban Waste Water Treatment Directive which has driven investment in P removal over the last two decades applies to larger sewage catchments of 10,000 people or more. Many to most chalk streams do not have towns of that size on them, or if they do it will be in the middle to lower reaches of the larger river systems. Although its a good news story for P removal, it has tended to benefit the lower reaches of larger systems leaving the headwaters and smaller rivers behind.

Another clear pattern is the difference in % of STWs on a given river or in a given catchment that feature P removal and the designated status of the river. All chalk streams are a Priority Habitat, but some are also SSSIs (Sites of Special Scientific Interest) – the Frome, Test, Kennet, Nar and Driffield Beck and some are SACs (Special Areas of Conservation) – the Itchen, Avon, Lambourne and Wensum. These two designations are much more powerful. SSSI and even more so SAC protected status has driven an investment in P removal in smaller-scale sewage treatment works which is clearly reflected in the WFD assessments.

That is not to say that our SSSIs and SACs are perfect. They clearly still suffer phosphate issues as some charts I will publish in due course show. But it does make the point very clearly that if we want to effect real change for all our chalk streams, right across the map from Dorset to Yorkshire, we need to look at the protected status of all chalk streams, not just the shining examples. Much of the anger and frustration felt by people who care about chalk streams is driven by the condition of the Cinderella streams that are still bedecked with sewage works (there are 175) which do not remove Phosphate. Priority Habitat as it stands is clearly not a sufficient imperative to action. Protected status is symbolically important, but more than that it drives investment. And investment is what these rivers so desperately need.

How Sensitive is Your Chalk Stream?

Above: the River Nar in Norfolk: a “protected” chalk stream not very sensitive to abstraction … apparently.

Did you know that in order to assess if a river’s flows support Good Ecological Status (GES) using the Environmental Flow Indicator (EFI) every river is now put in one of three Abstraction Sensitivity Bands (ASBs)?


The % figures in the table above indicate the reduction from natural flows deemed to be acceptable as a result of abstraction for the given river still to support GES. The Q number refers to the % of the time a given flow is exceeded: so Q30 equals relatively high flows and Q95 low flows. Flows in a high sensitivity river (ASB3) should deviate by no more than 10% below natural low flows.

The ASB banding is based on an assessment of three components: physical including gradient, catchment size, rainfall, and base-flow; macroinvertebrates; and fish communities.

One would expect, therefore, that all chalk streams would fall within the same ASB banding given that they are, almost definitively, of a type when it comes to the natural physical characteristics, which then define the macroinvertebrate and fish communities. In the original flow target matrix devised by UKTAG (from which the EFI evolved), all chalk streams fell within a single river type, divided between headwaters and downstream reaches.

So, I was surprised when I took a look at the ASB table to find that my local chalk stream, the River Nar, a protected site (SSSI) and arguably East Anglia’s finest chalk stream was in ASB1, the least sensitive to abstraction. This seems an anomaly to me.

But the River Nar is not the only river to have fallen into ASB1 in what actually appears to be a rather random assessment process. Can it really make sense that within a single chalk stream catchment we find rivers that are physically much alike spread across three different Abstraction Sensitivity Bands? The Wissey is one such case. But elsewhere, on the Frome say, we have the Wraxall and the Hooke (physically similar rivers) in two different bands. Likewise the Sydling and Cerne. If the assessment for ASB really is based on physical, macroinvertebrate and fish community assessments then this doesn’t make sense.

Below is a link to a table I have made of the ASBs for every chalk stream in England. Check it out to see what ASB your local chalk stream has been given. You can read the table by looking for the catchment your river is in. For example Thames / Colne etc. It starts at the River Bride in the southwest of Dorset and proceeds north east to end at the Gypsey Race in Yorkshire.

I suspect that those chalk streams in ASB1 are there by honest mistake: they are all in Kent, Norfolk, Lincolnshire and Yorkshire, in lower gradient catchments where the chalk reaches of rivers glide into fen-like rivers and often the waterbodies are lumped together. So, perhaps someone somewhere thinks the Nar is a cyprinid river because the waterbody division between the upper and lower river is in the Fens? Maybe. Even so, there are also a lot of chalk streams in ASB2 as well and I wonder why that is. In my opinion all chalk streams should be in Abstraction Sensitivity Band 3

Greater Protection for Chalk Streams in the Environment Bill

Sir Charles Walker MP recently sent the letter below to his colleagues in the All Party Parliamentary Group for Chalk Streams seeking their support for an amendment to the Environment Bill that would strengthen the protection of chalk streams by widening the definition of environmental damage to include damage caused by low flows due to unsustainable abstraction.

Can I encourage any of you who care about chalk streams and who live in a constituency with a chalk stream in it, to write to your local MP and encourage them to support Sir Charles’ amendment?

Next Steps for English Chalk Streams.

Redbourn Gauging Weir on the River Ver in May 2017: one of several Chilterns chalk streams still suffering from unsustainable abstraction. But maybe the tide has turned? Will the Ver be flowing properly again by 2027?

I’ve been asked by the Environment Agency, the Rivers Trust and CaBA to chair a national chalk stream restoration group whose first task will be to produce a national chalk stream restoration plan: it will be used to drive progress by government and regulators, water companies, landowners, NGOs and river associations right down to the grass roots level of individuals who are passionate about their local river … people like me.

So, a few weeks ago I was asked to talk about “Next Steps for English Chalk Streams” at the Chalk Stream Conference hosted by the Chilterns AONB. I discussed the same ideas earlier this week when Sir Charles Walker invited me to speak to the All Parliament Parliamentary Group of MPs for Chalk Streams.

I spoke about how there have been a number of action plans over the years and yet here we are … with our chalk streams still in crisis. I wonder what I can add to what has been said before, but will do my best. We are currently assembling a panel that is representative of the regulatory side of things, and of the water industry too and also of the national NGOs who have so long fought the good fight for chalk streams. I’m hoping the NGOs will drive ambition and that the regulators, and industry will respond positively to that vision. We will also draw on the expertise of a panel of experts in the field of chalk stream science, from hydrology to hydromorphology to ecology. Finally, we will share the drafts of our emerging action plan with a wide forum of stakeholders, from rivers trusts to grass roots river groups and individuals (I’m assembling that list and if you’re reading this and involved in a chalk stream advocacy group you’re probably on it, but drop me a line just in case).

You’ll see below in the words of my talk that I am keenly aware we need action above all else, that we have had action plans. But I think there is room to take stock of where we have got to and to analyse why certain key asks made again and again over the years have not been answered yet. I have some ideas as to why not.

Occasionally I have had a hand in previous reports on chalk streams: I helped WWF with their campaigns Rivers on the Edge and Flushed Away and with the 2014 State of England’s Chalk Streams report. But in terms of restoration plans I pulled together the plan that has driven our catchment restoration of the River Nar in Norfolk since about 2011. That’s the plan I can draw real life experience from.

And if I were to approach a national restoration plan as I have that local plan, I would say a vital first task is to break down the journey into assailable tasks, simply expressed. A lot of the ideas behind our Nar plan had been pulled together years before in the form of a much more involved and detailed plan that was brilliant in every respect other than in how it communicated its ideas to the places where they would actually make a difference: the general public. But that’s where the power to make change really resides. A growing sense that more and more people passionately care about chalk streams is what has driven the shift in tone and receptivity to change that we are seeing from our regulators right now.

Having worked out what needs to be done, we need to be strategic about the order in which we do things. Make a few early, doable and hopefully iconic gains and start to get a sense of the possible. I’d say we also need to be honest about the scale of the task, but not timid in the face of the realisation. We definitely need to be ambitious: but ambitious with an informed sense of what ambition actually means … and it doesn’t mean blaming everything on climate change, or publishing endless reports on systems-based thinking or coming up with more acronyms, or talking jargon behind closed doors. It means actually doing something, even if it’s a little thing and then doing another something, until all the little things add up to a big thing.

We’re not going to put our chalk streams back into excellent ecological health by next year, or the year after or even within five years and anyone who ever thought we could didn’t realise what was wrong with them. We’re certainly not going to do it by wishful thinking.

But river by river, reach by reach, we can do it. There are of course some things that could happen at a government and national level that would make a big difference: ending unsustainable abstraction as opposed to talking about ending unsustainable abstraction is one of them. Ending the scandal of CSO’s is another.

But maybe, just maybe this could happen … There’s been a surge of interest, lately, in the state of our chalk streams. All rivers being lovely, but chalk streams being potentially the loveliest of all. If you’re reading this blog you’ll already know that chalk streams are a very distinctive type of spring-fed river, almost unique to southern and eastern England. Between the beautifully named River Bride in Dorset and Yorkshire’s Gypsey Race there are just over 200 chalk streams ranging in size and character from the majestic River Test to hidden streams you’d have to almost fall in to notice, Lilliputian rivers with more lovely names like the Mel, Hiz or Gadder. They add up to most of the chalk streams in the world, a globally unique ecosystem that is ours to preserve and – because chalk streams have suffered greatly as south-eastern England has filled with people, business and industry – ours to restore to good ecological health. We seem to have really woken up recently to the duty we owe a natural environment that has suffered so much over recent decades.  But chalk streams feel to me like the ultimate test of our commitment. They are on our back door-step, they are struggling and they need protection. Are we up to it?

If so, what should the next steps be for our English chalk streams, steps that will lead them back to good health? Here’s my talk …

“As I often remind myself with regard to river restoration, to make good decisions about the future of a chalk stream, it is worth pausing to consider the past that has brought it to its current state. So, I hope you won’t mind if I spend a few minutes talking about next steps by talking about past steps. Because there’s a lot of frustration about what little progress we have made with regard to looking after our chalk streams. I want to consider why we are where we are, before offering some ideas on what we need to do next.

I fell in love with chalk streams many years ago, when I was given a Collins Encyclopaedia of Angling and found in it a picture of the River Lambourn: an ordinary little picture, nothing special, but one that captured that blowsy, English beauty of a summer’s day beside a brimful chalk-stream and which also captured my river-obsessed, childish imagination. 

Ten years later I moved to Dorset for my first job and fell in love with the wet reality of real chalk-streams. I started to fish them and to restore bits of them and this is where my exploration of the past begins, in the summer of 1995. 

Every day on the way to work I drove over a little chalk-stream called the River Tarrant. I got to know it very well. I rented half a mile of fishing from the farmer and would spend an hour or two there whenever I could. But this particular summer it started to dry, from the top down and from the bottom up, leaving a section in the middle full of magnificent trout caught in the trap of their drying river. 

The National Rivers Authority didn’t have the manpower to do much about it, but they did lend me some nets. So, as things got critical I went down there with my wife, Vicky, and together we netted the pools, rescuing the trout into buckets which we then put in the boot of my car and drove as fast as we dared down to the River Stour. Over a long week we rescued hundreds of trout against the clock of a vanishing river, until on the Saturday evening we had to stop, leaving one long pool only half done: it was late, and getting dark and we were totally exhausted. Vicky was heavily pregnant, after all. We’d get them the following morning, we thought. But by the next morning, it was too late. The pool had just vanished through the drying river bed, leaving dozens of trout, dead and dying in the mud.

I’ve sent my photos from that day to so many magazines that I have none left (slide film then and the magazines never return them) but I found this one online, taken by me in June 1995.

The River Tarrant had two abstraction stations on it: at the upper and lower ends of the river, exactly where the epicentres of the drying process had begun. I knew that abstraction had killed those fish, and I wrote angry letters to all and sundry saying as much. Vicky wrote an article in The Field

But in those days the official line was that abstraction didn’t actually cause chalk-streams to dry up or not enough to make the difference. Notwithstanding that I didn’t buy any of this – I did know that pulling trout out with nets and driving them around in buckets was not a sustainable way to protect them each time we had a dry summer. I also noticed that a pond beside the drying river had remained full all summer, simply because it was deep enough to pick up the groundwater that had deserted the perched river bed.

So, in between berating them about abstraction, I brokered a grant of £5000 from Wessex Water to excavate beside the river a series of inter-connected groundwater ponds into which I would put the rescued trout each summer and from which they could swim back to their chalk stream when the springs started to flow again. It was a no brainer scheme that couldn’t have but worked. But before I could get permission the Rivers Authority compelled me to commission a feasibility study including groundwater modelling. This study used up all my Wessex Water grant money and concluded that further research was necessary before we’d know the answer.

I learnt a lot that summer, about the issues facing chalk streams: the three-dimensional issues in the landscape, and the two-dimensional issues of the bureaucratic process which tends to complicate solutions to the point where nothing happens. You may have started to see how this story is a neat parable for explaining why, 25 years later, we are sadly still railing about unnaturally drying chalk-streams. 

First, we have long denied the cause of the problem. 

And then we have made the process of enacting solutions needlessly complicated. The latter sometimes to aid the former. And so it goes on.

In about 2006 I published a book on chalk-streams and drove through the Chilterns photographing them. With the exception of the Chess and Wye, I found mostly dry furrows in the landscape. 

In 2008 I helped WWF with their campaign against the over abstraction of chalk streams. We called it Rivers on the Edge. I wrote a speech which I gave bedside a dry chalk stream in Hertfordshire in which I said that our chalk streams were our burning rain forests, our melting ice caps. I hoped that idea would resonate and it did. 

The River Beane in 2008

Not so much, however, that in 2017 I was able to walk beside a River Beane with water in it. Whitehall was as dry in 2017 as it had been in 2007. And 1997 no doubt. As was the Hiz, the Mimram, the Misbourne, the upper Chess, the Ver.

The River Beane in 2017. At least it’s not overgrazed this time! But that pool of water … it isn’t flowing.

Will they still be dry in 2027? Dare we hope not? 

Because we have made some progress. The River Piddle dried often in the late 1980s, but now flows all the time, and as a result parts of it are close to chalk stream perfection. The River Og was bone dry when we campaigned for Rivers on the Edge. It flows again now. Other abstraction reductions have been made on the Chess, the Ver, the Darent. 

There are trout in London’s River Wandle when twenty years ago there were not. Some of our urban chalk streams are as clean as they have been since before the Industrial Revolution. 

River restoration is no longer seen by the authorities as a rogue activity pursued by eccentrics in smelly waders. It is encouraged and sometimes funded by the government to the degree that we can attempt projects now which were beyond our dreams twenty years ago.

So, there has been some progress and there is also now, I sense, a shift in mood. Nature and the environment are no longer niche concerns.

For my part I have been beavering away – quite literally, I suppose – on my local chalk stream in Norfolk and now I have been asked to chair a new national Chalk Streams Restoration Group, whose first task is to write a chalk stream restoration plan. 

Over the years I have to admit we’ve had a few of these. We’ve had in 2004 a report into the State of England’s Chalk-Streams. Then in 2009 we had WWF’s campaign Rivers on the Edge, followed in 2013 by the Angling Trust’s Chalk Stream Charter and in 2014 by a reinvestigation of the State of England’s Chalk Streams. And now most recently CRAG’s action plan. All good work. We kind of need another action plan like we need a hole in the head. What we really need is action.

But, if the momentum is here and there’s room for something that will help to catalyse that action, right now, I’m more than willing to have a go. 

Our end goal is healthy chalk streams we can all be proud of. It’s really very simple: there are three things that go to make a healthy chalk stream: 

water quantity, 

water quality, 

and good physical habitat. 

They are all interrelated, of course, and each one is shaped by the other. 

But the fundamental is water. Without water you have no river and in the case of the chalk-streams in London’s orbit, we frequently have no river. Or such diminished rivers that they are shadows of what they should be. This has gone on for too long, but one big reason why we have never quite cracked this abstraction nut is because the whole dark art of understanding abstraction and its impact on river flow has been hidden behind smoke and mirrors. Remember how I said that in the 1990s we were told abstraction wasn’t actually responsible for our unnaturally dry chalk streams? It seems incredible but only yesterday I bumped into a 1992 report on the upper Kennet that did just this, ascribing the cause to more or less everything but abstraction, wrapping the issue in a cloud of scientific radar chaff so that no-one without a PhD in geomorphology could disagree. 

I would like to see us democratise the knowledge. It’s not actually that complex. And knowledge, presented in such a way that it is easily comprehensible by the ordinary people who care passionately about their chalk streams, is what will harness the power to effect change. 

Right now you try getting hold of, let alone processing the information that will tell you the extent of the abstraction in a given valley, or the effective rainfall, or you try to understand the official assessment of whether a given river is over abstracted or not or even what actually defines unsustainable abstraction, and you’ll be set for a head-mangling exercise.

Instead, we should have a simple, national audit of the current abstraction regimes and rainfall data for all the English chalk-streams presented in an online map in an effortlessly comprehensible way, so we can see exactly what is coming in from the sky and exactly how much of that is not going out down the river? That would focus the debate, because it would put knowledge on both sides of the table.

And how about, in addition to the laudable but somewhat piecemeal abstraction reductions we have seen of late, giving ourselves a great big endorphin rush of an early hit by just getting on with the total no-brainer proposal of Chalk-Streams First? Chalk Streams First would amount to a total cessation of groundwater abstraction in all the chalk valleys of the Colne and Lea, a re-naturalisation of flows across the most beleaguered chalk stream region of all and for only a moderate loss to overall supply. It would offer us a model of how to do it elsewhere. What’s not to like? It’s such an easy win, the chance we have been waiting for to make progress on a significant scale.

As for water quality, remember that it took the Great Stink of the Thames to persuade the government of 1858 to invest in adequate sewer infrastructure. Perhaps it is a shame that we cannot, as Hercules might have, simply divert a river of Combined Sewer Overflows through Parliament. It hardly needs saying that the overuse of CSOs is a scandal. The Guardian recently reported the shocking headline figure that untreated sewage was released into English rivers 200,000 times in 2019. That Victorian sewage system catalysed by the 1858 stink is giving us problems some 170 years later. It is high time we updated it. To do that will be expensive, but Ofwat needs to understand that people care about not having poo in their rivers as well as their water bill.

Fakenham sewage outfall on the upper reaches of the River Wensum, one of only four chalk stream SACs

Further afield there is mounting evidence of the impact diffuse agricultural pollution is having on chalk-streams. We need to educate farmers, with independent, confidential advisory programmes backed up by warning and litigation if necessary. 

Over the road is a ditch and at the end of the ditch are the headwaters of the River Nar, one of only 14 SSSI chalk streams.

We also need to survey and fix the vast numbers of under-performing rural septic tanks, and sub-standard local sewage works.

Finally, and on to familiar territory for me, we need to lift our game when it comes to the physical restoration of chalk-streams. The degree to which they were dredged and canalised and the debilitating impact that has had on the bodily health of our chalk streams is poorly understood and massively under-estimated. 

A naturally meandering river, with an intact gravel bed and in-touch with its flood-plain is miles more healthy than a canalised, and entombed river, even if all the other factors are constant. It is perfectly possible, with funding and vision, to return our chalk-streams to a much more natural condition, even within the constraints of their man-made heritage. 

This is the River Nar taken out of its dredged and diverted course and put back down the centre of the floodplain. The old channel is now a spring-fed backwater full of sticklebacks and dragonflies. The new channel is full of current-loving chalk stream species like ranunculus, trout and riffle-dwelling mayflies and stoneflies.

To me this is the perfect chalk-stream: wild and unkempt, running free, but somehow also a palimpsest carrying the history of their mills and water-meadows and river-keepers.

Wouldn’t it be great if every water company were to adopt a chalk-stream as an exemplar of what is possible and work with the Rivers Trust and river associations to deliver a series of full catchment restorations, addressing that simple trinity of water quantity, quality and habitat as imaginatively and ambitiously as possible? 

Affinity Water and Thames Water will soon – I hope – meet this challenge on the River Chess. Will the other water companies join them and rehabilitate rivers like the Allen, the Cam, the Stiffkey, the Great Eau, the Foston Beck? Trojan Horse schemes on rivers like these would set the bar, showing beyond doubt that we can, with will and determination, find room for wild chalk-streams in our busy landscape, and through that enrich our lives immeasurably.

Thank you.”

The Index of English Chalk Streams

Good to see that the official Priority Habitat Map for chalk streams is receiving some attention, in part in order to include additions I made to the original 1999 Environment Agency map which listed 161 chalk streams (this map was used in the State of England’s Chalk Rivers published in July 2004). My version of a revised index was published in my anthology Chalk Streams with Medlar Press in 2005 and then later – with the helpful input of Dr Haydon Bailey – in revised form in the 2014 WWF State of England’s Chalk Streams Report. That list (which isn’t exhaustive, I’m sure) ended up at about 220 named rivers.

This is the link to my 2014 index, a version of which was in the WWF report.

The new guidance says the original 1999 list did “not provide adequate coverage of small chalk streams in headwater areas, including seasonally flowing winterbournes” which are important for biodiversity and deserve protection. I agree, although in fact what the original list mostly missed were the numerous scarp-face chalk streams that rise along the spring line on the north-east facing edge of the chalk massif. It also mixed up a few of the rivers in Yorkshire listing some several times with different names (because the local convention is for the river to take the name of the parish it is flowing through) and some not at all. In addition there were a few tiny little streams here and there which were missed, probably because you’d have to fall into them to find them. All these were easy omissions or confusions, but being a chalk stream nerd I could see the 1999 list was incomplete in the two areas I knew really well – Dorset and Norfolk – and started working on improving it.

For my 2005 version I used OS maps and driving around the country looking off bridges. For my 2014 version I had the advantage of online satellite maps (making looking off bridges much easier and faster), a new online publication of highly detailed geological maps and a complete series of OS maps from 1946, maps which pre-dated the post-war land drainage works that can complicate things. I took the names from these OS maps where I could.

In 2014 I also, with Dr Haydon Bailey’s help, refined what we mean by chalk stream. The River Nadder – as any fule knows – is a very different river to its neighbour, the Ebble. And yet they are both considered chalk streams. In fact no chalk stream is exactly like another, but as I went through the physical differences one river to the next, Haydon was able to help me group them into geological types.

I feel we need to move away from the too vague statement “any stream or river that has a flow regime dominated by natural discharges from the chalk aquifer should be included on the map” – (which could arguably include the Thames, or Ouse?) and towards the groups of chalk stream type as proposed below, because although these ideas might need some refinement, it is a more helpful and precise way of understanding what makes a chalk stream a chalk stream and what causes their subtle differences, one river to the next. Ultimately, this grouping could well help refine restoration and conservation strategies (and designations?) by river sub-type?

When we think of a chalk stream we think of a river of a certain size – medium to small mostly, though the lower Avon is a large river that preserves it’s chalk stream character almost all the way to the estuary – that is clear-watered most of the time; that is equable in its flow patterns – ie that isn’t ‘flashy” in its response to localised rainfall, but rather has a distinctly seasonal flow regime, at its highest in spring after the winter recharge, falling away through the summer and early autumn, before building again through the winter; that flows close to bankful most of the time, with in-river weed-growth bulking up flow volumes through the summer; and with a channel form that reflects this spring-fed flow regime – wide, shallow, gravelly, stable (the cross-sectional channel shape of a river is largely determined by the ratio of high flows to low flows: the higher the ratio the more incised the channel, and so chalk streams tend not to be that deeply incised).

If you know your chalk streams you’ll know that the Itchen fits this bill to a tee, but that the Nadder veers away somewhat, is more flashy, colours after, and is immediately responsive to, localised rain and is more naturally incised. It’s still a chalk stream – by reputation and according to our definitions – but maybe more a 9 carat plated chalk-stream than 24 carat solid. The difference is all down to subtleties of geology.

What makes a chalk stream a chalk stream, what gives the stream these characteristics as outlined above, is particularly the fact that the chalk body feeding our chalk streams lies very close to the surface, and the rivers which rise from it are not much influenced by superficial surface deposits: although some are more affected than others. This particularity in turn relates to the geographical relationship between the chalk body, and the limit of the last glacial maximum and the action of the glaciers and explains why there are chalk streams in England and Normandy, but not really anywhere else, in spite of the fact that there are great plains of chalk across eastern Europe. Basically the “cleaner” the chalk body from which the stream flows (ie very thin topsoil and not much in the way of superficial geologies or layers) the “purer” the chalk stream. The influence of other geologies will take a given river away from that purest expression which is typified by, say, the upper River Itchen or Test. Given that almost all chalk streams bump into other geologies somewhere along their route, this begs the question, when is a chalk stream not, or no longer, a chalk stream?

The River Nar, for example, flows for half of its length (the lower half) across the Fens, but gathers hardly any extra flow throughout that lower course – a few small tributaries which are also chalky in origin. For a large part of that lower course the channel is essentially man-made: in prehistoric times there wouldn’t have been a river so much as a freshwater segueing to salt marsh. Is that lower river a chalk-stream? I think so, but it is a sub-type.

The Fontmell Brook, as another example, rises off the scarp slope of the downs between Blandford and Shasftesbury, and for a few hundred yards is the prettiest Lilliputian chalk stream you could imagine, but then it drops onto sandstones and Gault clay and though it is always a lovely little river, its lower course is much more incised and moody: if you looked at it near Marston and knew nothing of its origins you would never describe it as a chalk stream.

By contrast the River Nadder and its numerous headwater tributaries flow across a mosaic of chalk and also Gault formation sandstones and mudstones before it squeezes through the pure chalk hills nearer Salisbury: it gets more and more chalky the further downstream you travel.

What this suggests is that the definition of a chalk stream is not binary: it is rather a spectrum condition with a suite of characteristics which fade the more a river is influenced by other geologies and geographies than the pure chalk downs.

Anyway, this is how we grouped England’s chalk streams:

Group A comprises streams that rise directly from the chalk, flow over chalk and subsequently flow over younger Tertiary (sand and clay) deposits. This group would include the majority of the Hampshire Basin Streams and the majority of those which flow in to the Thames Basin. These tend to be the slope-face streams and are generally longer than scarp-face streams. Note that most of the iconic chalk-streams like the Itchen or Test or Kennet are in this group.

Group B comprises streams which rise beyond the chalk and subsequently flow over / through the chalk – a minority of streams but the Great Stour in Kent is a good example, rising on the Gault clay / Greensand and then flowing through the chalk. The Nadder is another example, as is the Hampshire / Wiltshire Avon and the Dorset Frome. These streams will have less equable flow regimes than Group A streams, will tend will colour after heavy rain and take longer to clear too. The flow regime makes these rivers subtly more deeply incised in the landscape than the classic Group A streams.

Group C comprises streams rising from chalk which was directly impacted by major glacial action during the Pleistocene Ice Age. This would include some northern Chiltern streams and the East Anglian, Lincolnshire and Yorkshire streams. This chalk is more compressed and fractured with higher transmissibility than further south. Group C could be further subdivided into streams which flow from chalk over glacial outwash deposits and those that flow from chalk onto older (pre-glacial) river deposits, such as the pre-glacial Bytham River which flowed eastwards from the Midlands across Norfolk and emptied into the North Sea north of Lowestoft.

Group D comprises the scarp slope streams which all tend to run for a very short distance over older (clay rich) chalk and then flow out onto the underlying Gault Clay and Greensand beds. The Fontmell Brook and Iwerne stream in Dorset are scarp-slope streams, as are the streams north of the Chilterns, the westward flowing streams in north-west Norfolk, and all the streams east of the Yorkshire Wolds.

It seems that I listed a few streams which are proving tricky to find:

My index was arranged so that you could see where a given river was on a river system. The main river is the lead name and then the tributaries are indented below it, with the uppermost tributary listed first.

So the Bassingbourne is a tributary of the River Rhee (which is easy to confuse with the Cam because both the Cam and the Rhee seem to have interchangeable names on the map: even on the latest incarnation of Apple’s “Maps” the Rhee which rises at Ashwell Springs suddenly becomes the Cam after it flows under the Northfield Road). Anyway, to the west of Bassingbourne is a street called, tellingly, Brook Road and this is the river flowing under it:

And this is the geology that it flows from and across:

The springs are quite obvious in satellite images to the south but weirdly, the river does seem to vanish into a network of drains to the north of Bassingbourne.

The Binham Stream is actually a fairly obvious tributary of the River Stiffkey, in Norfolk, that flows west from Binham towards Warham.

The Bullhill Stream is a tributary of the Allen River (the Wiltshire Allen), a tributary of the Avon: it rises east of Cranborne and flows north-east through Lower Daggons and Bullhill. To be honest, I’m not sure it counts as a chalk-stream as the geology in that area is very mixed. The Allen River that it flows into is much more unambiguously a chalk stream.

The Crichel Stream is an obvious tributary of the Dorset Allen that flows down through Moor Crichel to Crichel Lake. See the screenshot below (all screenshots thanks to the miracle of Google StreetView and used here for the public good!)

The Gowthorpe Beck is a tricky one because of that Yorkshire habit of naming rivers by the parish: it’s also called Garrowby, Awnhams and Fangfoss! It’s also tiny and probably ephemeral and it’s only chalky at the foot of the downs. It’s just north of the A166 near Bishop’s Wilton in Yorkshire. This is a picture of it from the air:

The Iwerne Stream is unmissable: its a chalk stream that flows through Iwerne Minster, the next village and chalk tributary south from the Fontmell Brook. Look for Watery Lane! See pic below.

The Melbourne is another tributary of the Rhee but you can’t miss it if you find the village of Melbourne. It’s also called the Mel, so that can be confusing. See pic below.

The Otby Beck is another tricky to find, ambiguously named scarp-face chalk stream, tributary of the Ancholme, just north of Walesby, the next scarp-face stream north of the Rase in Market Rasen. Here’s a picture of it under Park Road on the way from the A46 to Claxby Park.

The River Chalgrove is easier to find. Just look for Chalgrove in Oxfordshire. It is made up of three small, scarp-face tributaries that rise in Lewknor, Shirburn and Watlington.

The Wyn is the next tributary downstream from the Tadnoll on the Dorset Frome. It flows over a mixed geology from Winfrith Newburgh and downstream, but it rises on the chalk near Chaldon Herring.

The Walsham is a tributary of the Little Ouse that rises at Walsham Le Willows in Norfolk. It flows over a very mixed geology that includes chalk, but it may well not quite count as a chalk stream. It is incised and clearly ephemeral, but has some nice meander patterns here and there.

The West Compton Stream is a tributary of the Frome, rising in the chalk hills of West Compton (south west of Wynford eagle and Maiden Newton) and looks like this:

The Wraxall Brook is also a headwater tributary of the River Frome, rising in the chalk, mudstone and sandstone formations of west Dorset near Rampisham. Although it flows over a fairly mixed geology it is definitely a chalk-stream and it picks up dozens of chalk springs along its route. See pic below.

The Beachamwell Stream is hard to find but it is a tributary of the Wissey, the next chalk tributary downstream from the Gadder which flows past Oxborough Hall. A lovely little chalk stream, it rises just south of Beachamwell and flows south west under the Gooderstone and Eastmoor Roads. See pic below.

I can’t find a Bishop Stream in my index, but there is Fonthill Bishop Stream, a tributary of the Nadder in Wiltshire which is very easy to find if you look for Fonthill Bishop. There is also a Bishop’s Wilton Beck, which is also easy to find if you look for Bishop’s Wilton in Yorkshire. It is small, I have to admit, as you can see:

The Charlton Marshall Stream is very hard to find. It is a tributary of the Stour near Charlton Marshall, in Dorset and by reputation was once an important salmon spawning stream for the main river. It is only a few hundred yards long, rising at the foot of the downs to the west of Spetisbury CE Primary School. I have a good photo of it somewhere as I go fishing near there every year. I’ll try to find it.

It was really only the name which made me think the Fulbourne must be a bourne. It is clearly ephemeral as the images on google maps show a dry stream bed, but the surrounding geology is definitely chalk. It is a tributary of the Quy water and rises in Fulbourne just to the east of Cambridge.

The Gussage Stream is another tributary of the Dorset Allen. It flows from Cashmoor through Gussage St Michael and Gussage All Saints. You can’t miss it really. See pic below.

The Kneeswell Stream, by contrast, is very, very hard to find. It’s near the Bassinbourn (see above), rising from springs at the base of the the same low chalk hills in Cambridgeshire, in the village of Kneesworth.

The North Bourn is a tributary of the Great Stour in Kent. Look for Northbourne near Shoulden. It’s chalk springs feed a veritable maze of lowland dykes but if ever there was a site for the restoration of this type of minor chalk stream it is here. In the picture below you can see the original river flowing through a drained field, with the ditches that now cary the water to either side. If the locals want to restore this, I’d be happy to help!

The Pakenham Fen is another chalk derived, fen-like river (perhaps we need a category of chalk stream that captures these rivers as there are lots of them) which rises near the Walsham (see above) flowing through Pakenham to Ixworth and into the Black Bourn and then the Little Ouse, in south Norfolk. See pic below.

The River Shep is a tributary of the Rhee (also called the Cam, but not THE Cam!). It flows through Shepreth. See pic below.

The Sapiston Brook is also known as the Blackbourn and it is the river into which the Walsham and the Pakenham Fen flow. It then flows into the Little Ouse. See pic below.

The West and East Hendreds, also called the Lockinge, are scarp face tributaries of the Thames that rise at East Lockinge, West Ginge and East Hendred to the east of Wantage. See pic below.

Finally, the Whitewool Stream is a tributary of the Meon that flows through the Meon Springs fishery from just north of Coombe.

Invasive Crayfish – new research, what we now know and what we still need to know

Photo Credit Roger Tabor Wikimedia Commons

Newly published research using a novel “triple drawdown” (TDD) technique for surveying has shown that signal crayfish can exist at astonishing densities – over 100 individuals per square meter counting juveniles. In the face of this revelation, the research also suggests that trapping, as a method of control, is relatively futile: something the scientific community has been saying for some while. The three study reaches were trapped in a conventional way, before being surveyed again using the TDD method. The numbers caught using TDD showed just how many conventional trapping left behind. 

The headlines so far on social media, including from the Angling Trust, have focussed on this latter point, the relative inefficacy of trapping. In fact I believe the study and some of the data suggest a more nuanced position on the efficacy of trapping as a means of control. At the very least it highlights an important area for further research if we are to limit the destruction by crayfish of our rare chalk streams.

My issue with the bald “trapping is futile” position is threefold: 

• there is currently no form of biocidal or allowable genetic control and yet signal crayfish cause enormous damage to the physical structure and biodiversity of globally rare chalk streams like the River Bure in Norfolk – we ought therefore to be looking for a form of trapping that can help control their numbers, even if outright elimination is currently impossible (the analogy with the government’s current Covid policy is too obvious to miss)

• the so-called futility / ineffectiveness is rarely contextualised against a desired outcome as above. If numerical control of the adults which damage the habitat were the desired goal then a form of trapping may well be found to be “effective”: this is something that has not yet been tested with a bespoke study programme.

• unlicensed, ad hoc and recreational trapping is likely to be one of the major vectors of the spread of invasive crayfish from one waterbody to another: the prevention of this spread is a major motivation – and a very laudable one – behind the message that trapping is futile as a means of control. But it may also mean that the message is subject to a touch of confirmation bias.

First of all it is worth emphasising that this new study is mostly about a novel method of sampling crayfish numbers, one that is far more effective at revealing the numbers of animals that exist than previous techniques. The literature review points out the deficiencies of all previous methods of trapping, electrofishing and even biocidal control, none of which get close to revealing the true numbers in the way that the triple-drawdown (TDD) method does.

It’s a brilliantly simple idea: the authors built a dam across the stream and used a pump to bypass and thus dry out a reach of streambed below the dam. The dried-out reach was then searched by hand, after which flow was allowed to resume, encouraging any crayfish that had managed to hide away during the first search to re-emerge. The process was repeated until no more crayfish could be found. Stop nets at the upper and lower limits prevented other crayfish from entering the study reach when it was re-watered.

The results of the TDD survey were compared with hand-searching and baited funnel trapping carried out on the exact same study reaches before the TDD trial.

The study was conducted on a small stream in Yorkshire, the Bookill Gill Beck in the Ribble catchment: a small stream, 5km long and 0.5 to 2 meters wide. This stream once held juvenile trout and salmon, as well as native crayfish. Now, following an illegal introduction of signal crayfish in 1995, all it seems to hold are signal crayfish in enormous numbers.

Three sites (DGB, PAD and CON) were selected for the study. In 2016 DGB and CON were surveyed and in 2017 DGB and PAD were surveyed. CON, therefore, was surveyed only once.

DGB and PAD were close to the each other quite high up the stream while CON was very close to the confluence with the Long Preston Beck.

The results unequivocally show that trapping tends to catch the larger individuals (average carapace length of approx 30 to 40mm) and that very few sub-adults were caught in the traps and virtually no juveniles. It’s worth pointing out that sub-adult crayfish are still sexually mature.

The hand-searching, on the other hand, caught roughly the same number as trapping (883 versus 721) but of a smaller size, mostly juveniles and sub-adults.

The TDD method, by contrast, caught vastly more crayfish and of all size classes, a total of 4,803, and revealed that a very large proportion of the crayfish population comprised juveniles, averaging 55%, ranging from 36% to 72%. 

These are the crayfish not caught by trapping, seeming to suggest – as the conclusion states – “unequivocally that trapping cannot be used as an effective control method for invasive crayfish populations at least in conditions resembling our study system”. Note that caveat to the “unequivocal” conclusion, italicised by me.

The River Bure, the river I get so animated about, (and in fact most chalk streams beset by signal crayfish), does not resemble the study system. The study system (at least the upper two sites accounting for five out of the six surveys) is devoid of predatory fish. It would have been a spawning tributary once upon a time, perhaps it still is. But when one appreciates that at the upper sites there were up 100 crayfish per square meter (!), it is easy to see why there aren’t any juvenile trout and salmon in the Bookill Gill Beck anymore.

Taking a longer and more detailed look at the findings and especially if we compare the CON survey site at the confluence (the only site where there are predatory fish, surveyed only once in 2016) with the upper sites, we see a more nuanced picture with regard the effectiveness of trapping as a means of control.

The authors of the report state that TDD gave a very good picture of the real numbers of crayfish in a given reach, and it does indeed seem as if this novel method of surveying surpasses other methods giving a more reliably “robust and representative information on the signal crayfish populations including estimates of density, biomass, male:female ratios and size‐class distribution”.

The total number of crayfish at the CON site, therefore, we can take as probably close to the TDD result of 538. Of these trapping caught only 75, roughly 14%, of the total number. 

If we look closer at the numbers we can see that:

• 37% of the crayfish caught in traps at CON had a carapace length greater than 35mm. Roughly 28 individual crayfish.

• whereas only 3% of the crayfish caught by TDD at CON had a carapace length greater than 35mm.  Roughly 17 individual crayfish.

• 62% of the crayfish caught in traps at CON had a carapace length of between 26 and 34mm. Roughly 46 individual crayfish.

• whereas 12% of the crayfish caught by TDD at CON had a carapace length of between 26 and 34mm.  Roughly 65 individual crayfish.

Trapping caught virtually no crayfish below this size, which TDD revealed to be 85% of the total number present.

In other words trapping caught hardly any juvenile crayfish, but it did catch significantly more large crayfish (40% more) even than TDD and what would appear to be 70% of the medium-sized adult crayfish, suggesting that trapping is quite effective at removing adult crayfish and very effective at removing the largest crayfish.

The results from the upper sites bear the same thing out, certainly as far as the larger crayfish are concerned. In 2017 trapping accounted for 153 large crayfish at PAD (82% of 187) whereas TDD accounted for 65 (5% of 1319) and it accounted for 90 large crayfish at DGB (38% of 236) while TDD accounted for only 13 (1% of 1290). 

For medium sized adults TDD is more effective: again in 2017 trapping accounted for 24 medium adult crayfish at PAD (13% of 187) whereas TDD accounted for 132 (10% of 1319) and it accounted for 134 at DGB (57% of 236) whereas TDD accounted for 425 (33% of 1290).

It might be more accurate to say, therefore, that trapping is ineffective as a means to control smaller and especially juvenile crayfish numbers, and therefore as a means to control sheer overall numbers. On the other hand it does appear to be potentially effective as a means to control larger crayfish – within an isolated reach at least.  

It is also worth taking a look at the crayfish population demographics of CON versus the other sites. It is notable that the raw density of crayfish at CON was a quarter that at DGB and half that at PAD, entirely because the numbers of smaller and juvenile crayfish were much lower. In fact in the year CON was surveyed, 2016, the number of juvenile crayfish at DGB was 1192 (72% of 1656) versus 193 at CON (36% of 538).

As the report states: “what is clear from all sites is the large number and overall dominance of juveniles in all the populations (36%–72%), with the relatively smaller population of juveniles at CON2016 potentially linked to greater predation pressure from fish”. 

As stated CON was the only site of the three where predatory fish were present: bullhead, Atlantic salmon (parr presumably), trout and eels. Predation at the upstream site would have been limited to otters and heron and the vast numbers of crayfish there, exceeding any densities heretofore recorded “could represent highly successful populations thriving under potentially optimal conditions.”

The italicised caveat in the concluding sentence I quoted above is an important one therefore, in that the study site represented optimal conditions for a numerically enormous population of crayfish, dominated by juveniles, with no predation. Faced with these conditions, if the goal of trapping was to remove all the crayfish, trapping clearly won’t work: because trapping doesn’t catch the numerous smaller crayfish, some of which are sexually mature.

But what if, on the other hand, one were to make the conditions less optimal, and what if there were predatory fish present? The data seems to suggest that trapping is actually quite an effective way, perhaps the most effective way, to capture larger crayfish. It also seems to suggest that predatory fish keep the population of juveniles significantly trimmed, by as much as 80% based on the comparison between sites in 2016.

The wild brown trout of the River Bure are surprisingly small for a chalk stream. Anecdotally, they used to be of a higher average size. The older anglers remember when the wild trout seemed to average closer to a pound, than a half-pound or less. This doesn’t really surprise me given what the crayfish have done to the river. 

2009 on the River Bure … the signal crayfish were present, but not in such Biblical numbers. The wild trout were more numerous and in better condition. The river looked good too.

The Bure is deeply incised for a chalk stream: it is quite a flashy river and it flows over a periglacial drift of sand and gravel into which it has cut a deepish channel which has also been dredged. The crayfish burrow into these steep banks, causing the banks to collapse along a fracture line at the extent of the burrows. The river thus gets wider and wider each year and progressively fills with silt and mud, which the widening stream cannot wash away. This, in turn, inhibits the growth of the kinds of plants which thrive in swift flowing chalk-streams. This silt and mud also smothers the habitat of the invertebrates on which the trout feed.

The crayfish are now present in such numbers that they churn up the silt, battling for territory and foraging for food and digging burrows: the opaque water further inhibits the growth of the plants. Thus everything is going down in a vicious cycle and it is difficult in this degrading habitat and diminishing larder, for trout to grow to the size one would expect on a chalk-stream. The crayfish have become the dominant species and their destructive behaviour only makes the habitat more and more favourable to the crayfish and less and less favourable to the trout.

2019 … ten years later and the river is a muddy shadow of what it once was because armies of signal crayfish, unchecked by any form of control, are pulling it apart bit by bit, collapsing the banks and filling the channel with mud and silt

Is it possible to turn this around? To tilt conditions back in favour of the trout and allow them to become a significant predatory impact on the young crayfish?

Far from suggesting that trapping is futile on a chalk stream so beset by crayfish as the Bure is, a careful reading of the study suggests that trapping could form a useful component for a carefully targeted programme of control designed to reset the balance and save the river from inexorable decline.

While the new research does indeed show that trapping is futile if the goal is to remove all the crayfish, including juveniles, it also shows that trapping is actually quite effective at capturing larger crayfish. And it suggests that predatory fish can have a significant impact on the numbers of juvenile crayfish, reducing the population by up to 80% in the 2016 comparison.

If you add the two together it is clear that trapping and predatory fish could together make significant inroads into either end of a crayfish population, provided the effort was sustained and provided the reaches were isolated in some way.

Now what if you added to this pincer three additional modes of control?

First, what if you made the habitat less favourable? Crayfish love to burrow, but they hate silty banks and reed-beds and they can’t burrow into banks armoured with gravel and geo-textile. About fifteen year ago Hunts Green on the River Lambourne was utterly plagued by crayfish and just like the Bure, had grown wider and wider. The keeper, Bruce Wheeler, could trap hundreds of crayfish in a night. Now he’d struggle to trap a dozen in a week. The difference? He has re-profiled the banks and armoured them with sloping gravel. That’s all. He hasn’t trapped. He has simply altered the habitat and made it less favourable.

Second, what if you neutered the large males (which will have been successfully singled out by the traps) and returned them? The larger males are territorial and predatory and cannibalistic. Research conducted by Nicky Green in Somerset (Barle Crayfish Project) has shown that neutering and replacing large male crayfish, while killing and removing all the others, can have a significant impact on the overall population. That research has been echoed in France and Italy.

Thirdly, what if you also used refuge traps, the kind (not used in the TDD study) that are more effective at capturing smaller crayfish and especially berried females?

Unlicensed and unmanaged trapping is very likely a significant, perhaps even the major vector for the spread of the crayfish plague. We don’t want to encourage trapping of this sort. But to be honest the people who trap crayfish in an unlicensed and unmanaged way don’t read scientific papers on the efficacy of the method as a means of crayfish control. They are a different kind of problem requiring a wholly different kind of campaign.

Trapping as means of crayfish control would most likely be carried out by people who care deeply about the habitat they are trying to protect and are looking for a way to make a difference. It would be perfectly possible to licence any trapping programme with biosecurity conditions: for example by using traps dedicated for use on a single, named river and tagged as such.


While this new Triple Drawdown method is novel in itself, and reveals a much better way of monitoring crayfish numbers and population dynamics and while the study does indeed back up the long-held claim that trapping is an ineffective form of eliminating a crayfish population, it also, I would argue, suggests that a targeted programme of control could well yield habitat-saving results. It certainly suggests that a research study could be fruitful.

We need to bring the two sides of this debate together, to unite in our condemnation of unlicensed and ad hoc or recreational trapping and also unite in an effort to find something that we river lovers can do in the face of the crayfish plague, not just what we shouldn’t do.

A New All Party Parliamentary Group for Chalk-Streams

Chalk-streams are finally getting some attention. Minister Pow recently made a clear statement in the Commons saying how much this government valued our chalk-streams and intended to take their conservation and restoration seriously. 

It is very good news also to see that today Charles Walker MP and Oliver Heald MP have launched a new All Party Parliamentary Group of MPs dedicated lobbying on behalf of our chalk-streams.

I hope that our Chalk-Streams First initiative to cease abstraction in the Chilterns will be a key talking point for the MPs. This idea would yield a massive environmental gain at a modest economic cost. That must be an attractive idea for a government looking for ways to honour its intention to do well by our chalk-streams.

The Chalk-Streams First idea underlines that the starting point for any healthy chalk-stream must be water. Water is the stream in chalk-stream. Without it you have nothing: a dry riverbed that weeds over, a relic furrow in the landscape, a ghost. I have taken pictures of such places and know that without a bridge or some now incongruous “no-fishing” sign it is hard to show that a river should be there. And after a while it is easy to forget.

So, our first and most important battle is for water. Most chalk-streams are abstracted and many unsustainably so. That must change.

If nothing else the APPG MPs who care about chalk-streams will do well to focus the government’s attention on to this and force a change. We need new legally binding abstraction limits – not guidelines – to properly protect these rivers and we need to find ways to help water companies to abide by them. Chalk-Streams First is the start because it is the model of how we can all move forward together to a more sustainable future.

These new limits should not be based solely on flow, as they are – very haphazardly – at the moment. Looking at flow alone doesn’t protect the ephemeral winterbourne reaches of chalk rivers and anyway it is subject to such subjective interpretation. What percentage reduction of fully natural is acceptable? What is a fully natural flow at any given point in space and time, anyway? 

Flow is so variable and because it is so variable it is impossible to adequately police, or even understand, the reduction that abstraction creates. That’s why we have been arguing about it for fifty years and still argue about it. Because without sophisticated and expensive computer models it is very difficult to say how much less than natural the flow is. For example, there is a UK BAP (Biodiversity Action Plan) target for acceptable flow reduction in chalk-streams – somewhere between 10 and 15%. I wish! No river I have ever looked at in detail has its flow reduced by abstraction by such a small amount. None.

It would be better to set limits to groundwater abstraction as a percentage of the annual re-charge of the aquifer. This is a much simpler idea: it sees the aquifer and catchment as a bank account, whereafter the water credit and debit cycle is child’s play to understand and even to measure. 

What goes in is effective rainfall – the rainfall that gets through to the aquifer after lossses to vegetation and evaporation. What goes out flows down the river. Unless it is abstracted instead. In which case it is lost to the river.

All sorts of nuances notwithstanding, it is basically that simple. Not only has an unsustainable amount of water been diverted from rivers to abstraction across all our chalk-streams (it is not uncommon to find that abstraction is the same or greater than river flow) there have been years in the Chilterns when abstraction has even exceeded the re-charge of effective rain! It doesn’t take Einstein to see that if you raid a bank account of more than you put in you will soon be broke. As the chart for the Ver below shows, abstraction in this chalk-stream has historically been well over those UK BAP targets. In the mid 1980s it crept up to 45 Mld, or 56% of the average annual recharge of about 80 Mld. No wonder the river dried up. Even now Ver abstraction is running at about 27 Mld when it should be about 8 Mld.

WEG Project Phase 2 – works in progress

All Photos - 1 of 1 (9)

On the 5th August we began work on the first section of the Phase 2 channel: this is the main channel just to the north of the “Stage Zero” section described in my last post and coloured red in the whole project overview on the link:

CAcreCommon WEG Project Overview WEB

For an idea of scale from the top to the bottom of our project, we’re talking about 1.75 km. The line of the original river most likely ran about here or between here and the Stage Zero section to the south marked with the wiggling blue lines. This ditch marks the lowest point in the floodplain, but there is evidence of all sorts of drainage works and floodplain disturbance here … as we dig where we are finding upturned river beds here and there … so it is hard to tell exactly.


For context … downstream, this straightened ditch flows into a more obviously meandering channel, (the orange section) much modified, but now feral and overgrown (which in turn runs back into the main river at the d’stream limit of our project) … and this I’m certain marks the pathway of the original channel. This feral section is part way through an interesting process of self-generated rehabilitation (see the pic below) squeezing through overgrown willows and doesn’t need a hell of a lot doing to it. We will drop some old poplars here and there to create pinch-points and we will replace gravel in the more heavily dredged sections: but mostly this lower reach will be an exercise in letting the river do the work, especially once whole flow of the river is directed at it.

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To connect the new channel on the Common (the purple channel in the overview plan) with the lower section of original channel (the orange channel), we needed to improve the ditch (in red) and also make it large enough to take a portion of the flow (some will flow through the Stage Zero section). The work on 220 meters of channel took about eight working days. It wasn’t easy, digging into water and trying to shape banks and a river bed where the gravel was in scant supply. We won gravel here and there by pushing the meanders onto the undamaged, northern bank and we have now built some piles of excess gravel coming out of the next phase which we will bring downstream to build up the riffles in this rehabilitated ditch.

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The two pics above were taken in more or less the same place and only a few days apart. The water looks rather muddy at the mo, but that is just old ditchwater and not yet a flowing river. Look carefully and you can see how we have pushed the meander on to the northern bank, under which we found a seam of undamaged gravel, which we then pulled into the foreground to underpin the riffle marked by the post … which still needs a bit more gravel. All the spoil on the northern (right in photo) bank will be graded into the slope once it is dry.

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Another before and after, but looking upstream. Again, you can see how we have pushed on to the northern edge. The channel is a little smaller here than upstream as the intention is to push a proportion of the flow through the woods to hold on and enhance the rather lovely habitat that is in there (see previous posts). So this channel is about 4 meters wide, but narrower in places, 70cm deep, with a meander length of approx 25 meters. We have yet to dress it out with LWD. The intention is definitely that the river will do the rest of the work, knocking this channel about a bit with any luck, so that in a year or two it looks authentically natural.

More thoughts on Stage Zero designs on chalk-streams


On August 5th we started work on the 2020 phase of creating a new channel on the River Nar between Castle Are and West Acre: replacing a dredged, high-level leat channel running along the contour line at the edge of the floodplain with a meandering, shallow and gravelly channel at the centre-line of the floodplain and in sync with the gradient line of the valley.

One key aim of this WEG funded project has been to increase the “hydrological connectivity” between the river and the floodplain. The channel cross-sectional profile is based on the few reference reaches I have found, including a relic, unmodified “lost” channel at Emmanuel’s Common. The new channel is relatively wide and shallow (70 – 80 cm deep and 6 meters wide), designed to flow at bankful or overspill for some of the year. We are also cutting slightly higher ephemeral channels that will also flow at these times. In addition we’re not infilling the old channels: rather we’re setting out LWD or gravel bars to retain water levels in these old channels to create another type of wetland habitat, with some groundwater flow and the capacity to take a proportion of high flows: I see them as man-made oxbows of sorts.

Finally, I have incorporated into the design a short 250 meter section of so-called “Stage Zero” channel, partly to add to the overall variety of the project, partly to learn more about what happens with Stage Zero type treatments in a low-energy, chalk-stream setting, partly to retain what has happened naturally there anyway.

To learn a bit more about the theory of Stage Zero, please see my previous post, or better still the look up work of Colin Thorne, Paul Powers and Brian Cluer … but in simple terms, Stage Zero is a way to replicate the pre-historic river, characterised by multiple channels, braiding across the floodplain floor and it involves, for example, regrading the now modified, degraded valley and then leaving the river to do its thing in terms of re-establishing a more natural anastomosed channel form from scratch. You set the process in motion, but the river does the work.

In this reach we are simply diverting a proportion of the flow across a wet woodland area, where it will run unconstrained to reunite with the main stream after about 250 meters. In designing this Stage Zero reach and trying to figure out how to get it to work I have bumped into a few basic issues which I suspect will apply to most projects like this in an English chalk-stream setting. They are of a practical, rather than theoretical nature, but they might be interesting to others planning this kind of thing.

Key to getting something like this to work (and please bear in mind that I haven’t yet got this one “to work” … I’m just building it, and quite how well it functions has yet to be discovered) is how to handle the step up and the step down: after all, you are trying to lift the flow from the bed level of the river on to the floodplain surface (and then you have to let it down again). That sounds simple enough, but English chalk-streams rarely divide into large enough holdings to make this quite as simple as it sounds. Let’s say your channel is one meter deep and the gradient of the chalk-stream is 1 in 600: a sudden step-up of one meter will likely impound the flow a long way upstream. That could cause problems if the upstream land & river are owned by someone else, especially if they aren’t bought in or involved with the project. It could also cause problems in that an impounded channel is generally less desirable habitat: you might be robbing Peter to pay Paul.

The “Stage Zero” in question here is in fact partly replacing a serendipitously self-generated “Stage Zero” caused by a breach in the higher level mill-leat, through which the water is pouring across the floodplain through a willow carr. So the mill-leat has solved the issue of how to create that step, by gradually lifting the river bed relative to the floodplain, at the expense of a long, impounded reach, which the rest of our project is designed to replace. Nevertheless, mill-leats might well be a great way to solve the issue of handling the step-up and if I were looking for projects sites I’d think carefully about the potential for mill-leats to take care of this primary design issue.

However, as soon as we connect our new channel we will lose the advantage of the mill-leat and so I have had to come up with another way to get the water up on to the floodplain surface. I have designed a two-fold solution. The plan below should help make sense of the following description.

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For various other reasons the new, meandering channel has been designed to flow just off-centre of the floodplain along a seam of gravel which coincided with the desired bed-levels. I have used this to tease a small amount out of each of the 16 riffles upstream to create a bed-level at the main take-off point that is just a bit higher than it could have been: 21:60 (riffle 16 in the plan above) against a floodplain surface of 22:20, as supposed to the 21:45 to 22:20 in my first design. Now my step up is only 60 cm, instead of 75 cm.

Although the floodplain surface beside the river is 22:20, it is in fact a little lower than this slightly to the south (22:10) and if I then travel down the valley a small amount, I find a floodplain surface that is 22:00 … and I also bump into the flow currently coming out of the high-level breach (round about the letter O in ZERO in the plan). So, I have cut a low sill in the southern bank of the main stream (set at 22:05 or so) at the take-off point and a wide and shallow channel that fans wider still as it goes away from the edge of the river to coincide with the point where the valley is at 22:00.

Now, when the water depth in the main stream exceeds 45 cm (which I anticipate it will when the ranunculus beds establish downstream) a proportion of the flow will spill across this low-cut sill and fan away into the woods and the Stage Zero section. The proportion will vary depending on flow levels but the channel d’stream of the take-off point is slightly smaller than upstream (riffles 17 to 24), and I have a pile of gravel and large limbs of LWD sitting beside the river ready to fine-tune the split.

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All Photos - 1 of 1 (2)The images above show the excavation of and finished Stage Zero cut-off, with the main channel to the right and the willow carr setting of the Stage Zero channel back left.

As a secondary source of flow, I have also taken advantage of the need for the entire project to enhance the so-called hydrological connectivity in the floodplain: an arrow-straight and long since forgotten ditch runs down the centre line of the floodplain. We will modify this ditch into a secondary ephemeral channel and set the take-off so that a proportion of higher flows run down it. This secondary channel comes off the new channel much further upstream where the bed level is 22:00 (upstream of riffle 8), so it will have the head to drive water to the Stage Zero section. If the d’stream cut-off point doesn’t work for some unforeseen reason, then this one should. Either way, we’ll have a network of channels creating a variety of free-flowing habitats, without any undue impoundments.



All Photos - 1 of 1 (3)The drone shot above taken by the plant contractor Gary of GRD Sales shows the emerging main channel, with the Stage Zero cut off in the distance by the willows. The secondary Stage Zero feeder channel will come from enhancing the old ditch which runs along the left of the photo.

As for the step-down … well the existing serendipitous Stage Zero has started that process already and we’ll just let it continue (see the pics below). Interestingly, the process of channel creation is happening quite quickly at this lower junction, where the gradient of the step change is giving the energy required to accelerate the process: again maybe something to consider when designing other Stage Zero projects. It has been theorised that low gradient sections might be more suitable, but the lower the gradient, the lower the energy and therefore, I suspect, the slower the process.


The two shots below shows how the existing self-generated Stage Zero channel is cutting a stream bed at the downstream end and – interestingly – how the few gravels present in the upper layers of the floodplain peat have settled out to form an emerging river bed.



Stage Zero on an English Chalk-Stream?

Some (hopefully useful) thoughts on the potential for Stage Zero projects in English chalk-streams.

Stage Zero – What is it?

Stage Zero is a new term to describe a very old landscape form: the way rivers used to be before we messed around with them. And we’ve been messing around with them since at least 3000 BC, and the Neolithic phase of deforestation, a landscape change so significant it is discernible in the pollen record and as layers of sedimentary deposits in our floodplains.

Coined by Brian Cluer and Colin Thorne to describe the pre-anthropogenic-disturbance condition of alluvial rivers, Stage Zero modifies Simon and Hupp’s 1986 Channel Evolution Model with the stage that precedes Stage One (for a more fulsome explanation look up the original paper, B Cluer and C. Thorne A stream evolution model integrating habitat and ecosystem benefits).

Essentially, Stage Zero describes a braided river system spread across the full width of the floodplain, its waters by turns flowing, seeping, rushing and gushing through a complex mosaic of wet woodland and grassland, timber jams, beaver dams and oxbows, the water-table fully saturated and the floodplain quite frequently and aptly, flooded. 

Stage Zero contrasts starkly with the highly modified, single thread, entrenched and artificially impounded river systems of our relatively developed landscapes, strait jackets from which our rivers rarely escape. Nowadays, at least in the UK, Stage Zero valley floors are rare as hen’s teeth, but one can find evidence of what our rivers used to be like: here and there in our landscape, and in old maps too and in satellite images from comparable but less developed landscapes in other parts of the world. 

Here, for example, is an aerial image of a spring-fed river I visited and fished on the Chilean / Argentine border back in 2000, a meandering multitude of channels, and oxbows spread across a floodplain of seeping wet grassland and bog: an unbelievably wild and special place. 

A spring-creek in a remote corner of Patagonia resembling Cluer and Thorne’s theorised Stage Zero

Colin Thorne makes the point in his paper that this type of channel form offers profound possibilities for enhanced bio-diversity and bio-mass and one only has to look at this Book of Kells of a river form, to see that the complex maze of habitats would likely harbour a wealth of aquatic life in its nooks and crannies. 

If its (non-indigenous) trout were any manifestation of that ecological richness, I can tell you that this river was rich indeed. But I’ve noted this again and again in the river systems I’ve explored in various parts of the world (often with a fly rod, but always with my eyes open as to what the habitat looks like and why): that the less disturbed the landscape, the richer the river. We’ve kind of lost sight of what rivers should be like back here in the UK. Even the good ones are not exactly running on all cylinders.

A spring creek in New Zealand

But can they ever? In our busy landscape? If the multitudinous channels and saturated floodplain of the proto river form are so indisputably richer than what we have inherited, the obvious and urgent question becomes: can we restore our rivers to this state – or to some acceptable version of it – and if so, where and how?

Stage Zero on an English Chalk-Stream?

Following the publication of Colin’s paper, the instigation of pioneering restoration work on rivers in Oregon, USA, (look up Powers et al, A Process-based approach to restoring depositional river valleys to Stage 0, an anastomising channel network) and the early results of that work in the form of numerous and fat juvenile salmon, there has been a great deal of interest in the possibility of restoring river systems to Stage Zero in the UK. A trial Stage Zero project is in progress at the National Trust Holnicote estate and others are at the planning and scoping stages.

I came across Colin’s research very recently when planning the next phase of our own catchment-based restoration project on the River Nar in Norfolk, a project I have been involved with since 2011. We have moved through various stages of work on the Nar and our projects have incorporated various channel rehabilitation techniques, tailored by site and budget constraints and by what particular problems we were addressing at any given reach. 

The first few years we were mostly working on those sections of channel that could be improved with cost-effective and simple rehabilitation work within the existing channel. We did a lot with Large Woody Debris (LWD), at least until we reached the places where LWD alone hits its limits of efficacy in addressing the fundamental problem: where the channel is entrenched, dredged and impounded, in others words. We found LWD just doesn’t cut the mustard when the mustard is a hollowed-out river flowing half-a-meter below where it should be, or an impounded river backed up by a mill.

Large Woody Debris on the River Nar

More recently and to tackle these specific issues much of the work has involved various types of channel recreation or recovery, through re-meandering, cutting brand-new channels and bed-level recovery.

A new channel in the upper catchment replacing the deep Napoleonic-era ditch which ran along the straight line of the trees
A new channel on the upper river, as above.
A new channel on the middle river, replacing a dredged mill-leat.

Right now we are half-way through a project designed to move the river from a higher-level impounded, dredged and unnaturally straight diversion back to the centre of the floodplain, to recover the “original” channel where we can find it and to re-create a meandering, relatively natural channel where we can’t and in both cases to recover the natural gradient of the valley. This is a Water Environment Grant (WEG) funded project on almost 2km of river. 

An Accidental Stage Zero

At the western end of our new project site, towards the lower end of the existing higher-level mill-leat, a breach formed in the man-made bank about five or six years ago. I didn’t think much about it at first, but recently I have been watching the results with some interest. The water is now pouring across the floodplain through a relatively feral patch of woodland from the higher level channel to the “original” channel (albeit this channel has also been heavily modified).

An accidental Stage Zero on the River Nar, formed by a breach in a mill leat.

As you can see, it’s a miniature Stage Zero project self-generated as a result of the decay of the old leat embankment. Sadly, the process will stop when we divert the river back to the floodplain as the water will be taken away from the higher level leat. I was starting to think about how I could recreate the same thing as part of the new project when a morphologist with one of the contractors we are using suggested I look up Stage Zero and this lead me discover to Colin’s work. 

I got in touch with Colin and he in turn put me in touch with Paul Powers who works on large-scale river projects in Oregon. Together they have helped me to plan the incorporation of a new Stage Zero reach within our overall project, where I will try to recreate what you see in the photo above.

The Geomorphic Gradient Line & The Evolution of the Post-Glacial Chalk-Stream

During that process Paul introduced me to the concept of the Geomorphic Gradient Line (GGL) and produced for me a GGL set of LIDAR tiles for the valley around the location of our project. 

The “GGL” is the fundamental gradient of the valley along the theoretical surface of the pre-disturbance floodplain: it can be deduced by observing the surface level of the disturbed floodplain – which will be full of man-made interruptions and irregularities – and averaging all the high and lows and in betweens, to arrive at a steady slope. In Paul’s Stage Zero work in Oregon he has literally re-graded or “re-set” floodplains by cutting down the highs and filling in the lows – including the entrenched river channels: the river is then left to begin the process of drawing itself from scratch across the newly stretched canvas of the floodplain.

As soon as Paul sent that document through for the River Nar near Castle Acre I saw an explanation for something that had been bothering me on countless field surveys, and it is this that I particularly want to share as I feel it is very relevant to the question of site selection for similar projects on many, if not most, English chalk-streams.

A clear step in the gradient line of the floodplain surface is visible at this culvert in the water-meadow system

The GGL tiles showed a really distinct set of steps in the valley surface. Paul had noticed them and pointed them out, but from Oregon he could only guess at their origins. I knew right away: the steps were caused by Domesday Mills and water-meadow structures. They explained why, as I had walked back and forth across the floodplain planning the next phase of our project, I had failed in some locations to find gravel at the expected depth under the floodplain surface, whereas in other places the gravel was exactly where I thought it should be.

What do I mean by “expected depth”? Imagine the phases of evolution of a chalk-stream since the last glacial maximum: we must start with a canvas of bare and recently defrosted chalk downland and in the valley floors a barren glacial outwash of sand and gravel with springs oozing up more or less all over the place, the water slowly accreting into channels: like the channels mazing across a coarse, sandy beach when the tide goes out. The powerful forces that carved the valleys have retreated from the landscape, leaving behind ultra-low energy river systems, fed by aquifers draining the absorbent hillsides. In a warming climate vegetation becomes the defining force shaping the floodplain surface and the maze of channels flowing across it: the valley floor accretes, not so much because material is washed down onto it from the hillsides, as because vegetation is growing and dying, year after year, on the floodplain itself. This would explain the distinct layering common to chalk-stream floodplains of pure peat with some sand but a very low gravel content, above a hard valley floor of almost pure gravel and sand, with very little detrital content.  

Of course, no two valleys are the same and no two parts of the same valley are quite the same either. The peat depth will vary and the slumping outwash of sand and gravel underneath is marbled and furrowed. I know this is true on the Nar at least because in order to plan the routes of the recreated meandering channels we have cut I have surveyed the valley floor with – at one end of the sophistication curve – a mobile GPS surveying station, and – at the other – a road-pin, with a hazel handle wired across the top of it, which I used to the probe the depth of the gravel. I use these to make a record of the floodplain level and the depth of the gravel beneath it in order to build up some kind of underground map of the post-glacial valley floor. 

In other parts of the valley and at the upper reaches of this latest project site I have tended to find that the road-pin drives easily through the peat until it hits a hard gravel surface about 75 to 90 cm underneath the floodplain surface. This matches up with the measurements I have taken off a few reference reaches of relatively unmodified river channel, including a set of meanders a mile or so up the valley which were cut off by another mill-leat several hundred years ago and otherwise have not been modified, straightened or dredged. These relic meanders revealed a channel that was shallow and wide: 5 to 6 meters and only 75 to 90 cm deep from floodplain surface to gravel river-bed.

The relic unmodified channel on the River Nar used for reference dimensions for new channel projects: this channel had been abandoned for several hundred years but flow was restored to it in 2014.

A Domesday Legacy

However, as I moved downstream surveying the reach of floodplain where we will build the new project, I found that the gravel floor in the centre of the floodplain (where I was almost certain the river had once been situated) was more like 1.4 to 1.5 meters beneath the floodplain surface, and in places too deep to find with my 1.5 meter road-pin.

This gave me a problem for locating our new channel: I had either to cut a channel of the reference dimensions (5 to 8 m wide and 75 to 90cm deep) that would have a soft, detrital bed, or I had to cut a channel down to the gravel, but that would be unnaturally deep and incised.

It wasn’t a problem that went away with Paul’s GGL map, but at least I had a solid explanation. My working theory had been that neolithic forest clearances dumped a whole load of silt in the valley which had built up at pinch points to create swampy lagoons. In fact Paul’s GGL showed a more startlingly recent anthropogenic explanation, a watermill. But could that much material (50 cm) have accreted since mills were built? Maybe. Most of the mill-sites on the Nar are Domesday sites, which means that mills have been in those places since at least 1086, but probably a century or two longer than that. Call it 1200 years. Or a third to half millimetre of accretion per year, which seems plausible and long enough to have caused an uplift in the floodplain surface of about 40 to 50 cm, across the full width of the valley and for some distance – 1 to 2km – upstream of each mill site.

Unfortunately as far as Stage Zero or any other channel restoration project goes that gives us two diverging surfaces to think about: the steadily sloping surface of the hard, gravel/sand valley floor and the stepped surface of the floodplain that “should be” 75 to 90cm above it, but is actually up to 50cm “too high” in the anthropogenically-enhanced depositional reaches upstream of ancient water-mills and others structures. 

Most English chalk-streams will be similar to this, as they share the same basic geological and glacial building blocks (although there will be local variations depending on geology and glacial processes, the depth and type of the peri-glacial drift and the base-flow index of the river) and the same history of modifications and milling. But it won’t just be mills that complicate the picture: navigation and water-meadow modifications, common to almost all chalk-streams, will have added a similar layer of impoundment and stepped disconnections between the surface gradient and the valley-floor gradient.

How can we make Stage Zero work in our densely archeological mosaic of a landscape?

Stage Zero offers hugely exciting possibilities, but not exactly straightforward ones given the likelihood that the average chalk-stream floodplain is not only out of step with the post-glacial valley floor, but immovably so given how densely archaeological our chalk-streams really are. 

Certainly on the Nar we cannot even think about re-setting GGL of the floodplain in a way that would unite it with the GGL of the sand / gravel valley floor: we’d have to knock down mills that are now expensive houses, not to mention move millions of tons of material from designated Sites of Special Scientific Interest, and fill in an enormous gravel pit.

In working out how to make this kind of project a success it seems that there are two interlinked issues to consider carefully: the process of channel evolution and the way in which site location will influence that process. 

To what degree will the Stage Zero prescription – in any given location – mimic the way the chalk-streams were formed and thus recreate their original pre-disturbance state? How long it will take, given that chalk-streams are low-energy systems? Where do you locate a project site in our densely archaeological chalk-stream landscapes? How will the choice of location impact on the success or otherwise of the process?

The Stage Zero projects in Oregon have addressed issues of incision and channelisation where the river has become divorced from the floodplain by regrading that upper, floodplain surface over quite large areas, allowing the now freely flowing water to start a process of channel incision, creating eventually an anatomised plan-form across the width of the floodplain, resembling the original “pre-disturbance” state of the river. The Powers et al paper cited above lists the various Oregon projects all but one of which are rain or snowmelt systems and all but one of which are sited on steeper valley slopes than is typical of an English chalk-stream (0.2 to 0.1%). One is partially spring-fed, but that appears to be on a very steep stream (Three Mile Creek, with a 7% valley slope). One project carried out on open meadow on Whychus Creek, looks from photographs to have a lot to teach us about the possibilities for similar work on an English chalk-stream, but even so Whychus Creek has a 0.9% slope and is a glacial and snow/rain-fed system.

As far as I know, therefore, we have yet to see this kind of project carried out to maturity on a spring-creek like system of comparable slope, base-flow and geological underpinnings to English chalk-streams.

It must be fundamental to their formation, for example, that chalk-streams were not so much etched into a pre-existing floodplain, but rather they ‘grew’ from their glacial outwash foundations upwards, with the emergent vegetation shaping and defining the boundaries of the flowing water and vice versa. 

This aerial view of a glacial / rain-fed system (centre of photo) with spring-creek tributaries (the dark lines on each side of the floodplain) clearly shows the role an accreting floodplain and its vegetation play in shaping the planform of spring-fed rivers.

Of course beaver dams and fallen trees will have forced water to take new pathways, creating channels that were indeed ‘etched’ down through the surrounding peaty floodplain, but even then, in a pre-disturbance state, that post-glacial gravel floor would have been within reach of the emergent channels, flows were not denuded by abstraction, the gradient of the floodplain was not interrupted by mills and the floodplain was likely an open matrix of trees and tussocky grass.

The speed of the evolution of a project today and its results will be heavily influenced by these kinds of factors.  

The results of a Stage Zero project may vary considerably depending on the gradient of the site, the presence of immovable historic structures that impound the river and the potential disconnect between the floodplain surface and the post-glacial sand / gravel floor.

For example, a hypothetical Stage Zero project in an impounded part of the valley where the gravel floor is beyond the reach of the emergent channels, where there is a fundamental and immovable obstruction to the gradient across the width of the floodplain (as appears to be the case with ancient mills), where there aren’t any trees and where the land surface is grass pasture, will likely evolve into a wetland bog for a long time before it looks anything like a braided river system. Stepping the water up onto the floodplain surface will also most likely cause an impoundment to flows upstream of the project site too.

Whereas the same kind of hypothetical project downstream of an ancient mill, or where the gradient of the valley floor and floodplain are in sync, where there is a wooded and tussocky floodplain, would – I imagine – evolve into a matrix of emergent channels much more quickly: there, with the maximum possible gradient, a mosaic of shade and the structures of tree-roots, fallen timber and tussock grass to coral the currents, the emergent channels will not only etch into the floodplain more quickly, they will find gravel when they do so.

A good site for a Stage Zero type project: the mill leat upstream of the mill would provide the means to step the water on to the surface of the floodplain, and the floodplain itself is close to the natural gradient line of the valley.

If Stage Zero is an exciting prospect for some reaches of our English chalk-streams, we still need to go at it quite carefully, because the difference between one set of results and another – even if they might look equally attractive to someone zoned in on bio-diversity alone – could well be the difference between carrying the support of the wider range of stakeholder interests, or losing it.

In the end site location will be best achieved through a good knowledge of the river valley, its landowners and the various stakeholder interests, and then careful detective work into the history of a given site, and the gradient line not only of the floodplain surface but also of the post-glacial valley floor. Some places will be better than others and the ability of the river to work its magic will – I suspect – vary considerably in terms of speed of process and the type of river channel(s) that will result.

As for the project on the River Nar, In Norfolk, where we are working on that kind of problematic reach where the floodplain surface is too high, and somewhat disconnected from the gravel floor, I have designed a project that incorporates elements of Stage Zero throughout and one 250m experimental section where we will push the water out of the channel and across the floodplain through a wet-woodland area. I’m curious see what happens and how long it takes.

With Colin and Paul’s help I have sketched out a way of stepping the flow up on to the floodplain surface without impounding the upstream flow too much – I hope. We will actually cut a single thread channel (or in fact rehabilitate a ditch that is part of an ancient drainage / water-meadow system) and then block it with a pseudo beaver-dam by felling several large trees. This will back the water up into a pseudo beaver-pond area, the low point of which will feed the flow across a wide sill into the woodland that will be the locus of the Stage Zero and is the likely route of the pre-disturbance channel area. The flow will gather at the downstream end into the remnant of the original channel, which is in itself an interesting, once modified but now feral channel full of overgrown willows.

Above and below: the modified but now feral and likely original course of the river into which we will connect the proposed Stage Zero section.
The second phase of the Nar WEG project is at the rh side of the image, with proposed new channels replacing the higher level mill-leat to the south, restoring gradient, reference channel dimensions and connectivity between river and floodplain. The proposed Stage Zero section is marked (although the entire site will feature the possibility for the evolution of multiple channels). Note how the mill-leat and mill have caused a step-change in the GGL of the floodplain surface.

Upstream of this Stage Zero section we will be running a new, meandering singe-thread channel just off to the side of the centre-line of the floodplain, where the gravel is at the correct depth for the reference dimensions, but we will then use the turfs which we cut from the path of the new channel to patch out the low-lying points of the floodplain surface to create a series of ephemeral channels. The intention is then to dress out all these channels with Large Woody Debris to force the water to take multiple pathways, certainly in high flows, but to an extent in all flows – remembering that in chalk-streams with good gradient and dense ranunculus bed, summer flows are often “higher” than winter: higher relative to the floodplain surface.

Effectively then, we will have a mix of main and secondary channels. All this work will be carried out this summer. I’ll post updates here. I’ll be happy to show anyone the site, if that would be interesting to those planning their own similar projects.

The first phase in the WEG project at the upper end of the site, the new channel to the south (right), the old channel to the north. The lower half of the new channel is still impounded by the raised mill leat downstream but in the upper half (which is flowing well) there is additional LWD as shown in the photo below (aerial photo credit Aaron Mcdonnell @ Five Rivers Environmental Contracting).
The new channel after six months and dressed out with LWD.