Rivers Still on the Edge

Having lost, or at least misplaced, the files for the film I made for the WWF almost exactly ten years ago, I was pleased to find it still on YouTube.

You can watch the film, Rivers on the Edge HERE

The aim of this film was to highlight the chalk-stream abstraction crisis and suggest ways that we can all make a difference, not least by using less water. That last idea still stands – we need to be far more careful in our use of a precious resource – but I remember only hoping against hope back then that OFWAT would take an interest in the environmental impact of abstraction, that water companies would even acknowledge that abstraction denuded chalk-streams, that we would ever see a meaningful attempt to engage in the issue.

Well, things have changed and I’m happy to say that RAPID, the organisation set up to oversee OFWAT’s strategic review of water resources across the south east, is taking a keen interest in our Chalk Streams First idea. Thames Water and Affinity water appear to be engaging with it seriously too. Hopefully we (the Chalk-Streams First coalition*) will soon be able to say what shape and form the investigation of the idea will take and how we will remain involved.

We need to count in decades when it comes to the progress of our battle against the abstraction of chalk-streams. If anything, the fact that I made this film in 2009 and yet the drying rivers I walked along then were bone dry in the spring of 2017, just shows how desperately we need our Chalk-Streams First initiative to be taken seriously.

* Chalk-Streams First is supported by a coalition of The Rivers Trust, The Angling Trust, WWF UK, Salmon & Trout Conservation and The Wild Trout Trust : we are calling for the idea to be included in OFWAT’s multi-million pound strategic review of water resources across the south east.

Chalk-Streams First

Ten years ago, I worked on a campaign with WWF and made a film focussing on the terrible impact of abstraction in English chalk-streams. We called it Rivers on the Edge, because they were … on the edge of survival. In a speech on the banks of the River Mimram in the heart of the Chilterns I highlighted how locals there and on the neighbouring River Beane had been protesting about their drying rivers for at least twenty years. They still are. For too long it’s been Groundhog Day with our over-abstracted chalk-streams. But finally, we may just dare to hope that we can fix this problem once and for all, at least in the Chilterns.

It’s high time we did.

Chalk-streams are paradisiacal rivers. Their qualities of clear, cool water, equable flows, and abundant wildlife all derive from that qualifying word, chalk. We all know it from black-boards. Chalk is common enough geologically too: there are great swathes of it across eastern Europe. But the unique way in which the English chalk lies at the surface and was worn away but not completely worn away by the last Ice Age has given us eight-tenths of the global total of the rivers we know as true chalk-streams. The remainder are found over the channel in northern France. 

That’s some natural heritage. The unspoilt chalk-stream is a watery Garden of Eden. With their chequered beds of water crowfoot swaying in the marbled currents, their banks decked out in a bunting of marsh marigolds, water mint, and flag iris, they are utterly beautiful in a way that almost defines the southern English countryside. Chalk-streams are rich in wildlife too: under the surface there are brown trout and grayling, white-clawed crayfish, freshwater shrimp and all sorts of darting insects; in and over the plashy meadows there are snipe and otters, water voles and mayflies. Chalk-streams are an English Okavango, an English Great Barrier Reef, an English rainforest. 

Which ought to mean we should value this heritage as highly as we would any other globally-unique eco-system.

Sadly, we don’t. Or we haven’t. Instead these unique rivers are too often abused: some to the extent that they have dried up and ceased to be rivers at all. In May 2017 WWF commissioned me to take photographs of the same Chilterns chalk-streams we had mourned in 2010 … what was left of them at least. They were dry (again) or hardly flowing at a time of year when chalk-streams are usually at the fullest. The worst I’d ever seen. The rivers were dry, or mere trickles, far downstream of their winterbourne headwaters, far downstream of ancient mills, and old market towns and “No Fishing” signs and even Environment Agency flow-gauging weirs. 


In spite of, or perhaps because of, how bad it got in 2017 we can at least say that some progress has been made: no-one is denying there’s a problem anymore. No-one is questioning the link between abstraction and drying chalk-streams or suggesting that further research is needed before we can be sure. There have even been some moves to lessen abstraction. 

But the real problem at the heart of all this is that southern England is full of people and water is scarce. The Water Companies have an obligation to supply water to the public. They have a right to abstract it, and although nowadays the Environment Agency has the power to revoke licences they deem to be environmentally damaging, in reality alternatives to the water in the chalk aquifer are very difficult and expensive to realise. So, for year after year we make incremental progress without ever fixing the problem.

Until now?

A new idea called Chalk-Streams First has the potential to completely re-naturalise the flows in all of the Chilterns chalk-streams with potentially only a small net loss to overall public water supply. It is a scheme that could be delivered in the near future using as its basis infrastructure that is already planned for and costed in the water company management plans.

Chalk-Streams First is supported by a coalition of The Rivers Trust, The Angling Trust, WWF UK, Salmon & Trout Conservation and The Wild Trout Trust and we are calling for the idea to be included in OFWAT’s multi-million pound strategic review of water resources across the south east.

Thus far the proposal has been independently reviewed by expert hydrological engineer Colin Fenn whose key conclusion was …

“ … that the draft Chalk-Streams First proposition, as put, identifies a feasible and a viable solution to the problem of chronic flow depletion in the internationally-rare and precious chalk-streams of the Chiltern Hills; it being to allow flows in the upstream chalk-streams of the Chilterns to run unreduced by abstraction, with water being taken from the correspondingly enhanced flows in the downstream Colne and Lee, and as needs may be from a range of other less-environmentally fragile sources to meet the needs of demand centres in the Chilterns, using Affinity Water’s already planned ‘Supply 2040 scheme.”

HERE is the Chalk-Streams First proposal.

The Chalk-Streams First coalition is calling for an urgent, and detailed and fully independent investigation of the idea as part of OFWAT’s strategic investigation of water resources across the South East England.

It’s high time we put Chalk-Streams First.

How Chalk-Streams First Works

If Chalk-Streams First sounds too good to be true, it is also relatively easy to explain how it works. First you need to understand the relationship between the level of the underground body of water – the aquifer – and the flow in the river. It is both a very complex relationship – there are all sorts of nuances and no two valleys are the same – and yet a rather simple one which can boiled down to: the higher the groundwater, the higher the flow in the chalk-stream. There’s even an equation that is remarkably accurate across many streams: a 10% increase in the groundwater level equates to a 25% increase in the river flow. And as the groundwater level increases, so the chalk-stream rises further and further up the valley.

Chalk-Stream Diag

To illustrate it, let’s see the chalk aquifer and chalk-stream as a bucket with holes up the sides. Those holes up the sides represent the length of the river: the highest few holes are the winterbourne headwaters, and below them are the middle and lower reaches down to the bottom of the bucket. 

The bucket itself is the chalk aquifer. Now we can fill the bucket with a hose: the water coming out of the hose is rainfall. The water spilling out through the holes: that’s the river flow. If we turn the tap up really hard so that the bucket starts to fill: that’s the winter recharge period. If we turn the tap down so that the bucket starts to empty: that’s the summer discharge period. 

The real chalk aquifer rises and falls seasonally, just like this simplified model. Aquifers fill in the winter when inflow tends to exceed outflow (even if the main natural outflow is the river, a real chalk-stream valley has other forms of natural outflow … transpiration and evaporation and some movement of water through the chalk underground) and discharge over the summer months, reaching a low point in early autumn, before the winter re-charge period begins. Winter rainfall is key therefore: the chart below from the River Tarrant shows how important winter rainfall is for the replenishing of groundwater levels.

Tarrant Recharge

The real chalk-stream flows like this too. The flow increases as the bucket fills: just as the river flow increases as the groundwater builds in winter. The river (represented by the holes up the side) gets longer, too. And then as we turn the tap down through ‘the summer’ the holes at the top falter to a trickle and then one by one they stop altogether as the water level drops further. That’s the upper reaches of the river drying up and the overall flow decreasing, seasonally.

Notice how the water spurts farthest from the holes lower down the bucket and also as the level in the bucket falls during the summer discharge the flow from all the holes added together diminishes too. That’s because the flow rate is a response to the hydrostatic pressure in the bucket. The lower the level, the lower the flow: just like in a real chalk-stream.

Now to see the impact of abstraction … let’s set the tap so that all the holes are flowing and the water coming in from the hose matches the water going out through the holes. 


Then let’s drill another hole in the side of the bucket and create a new outflow that represents abstraction, with some of the water going in a different direction towards “public water supply”.


As soon as that hole is tapped, the bucket will start to empty until it reaches a new state of equilibrium at a lower level: that is the impact of abstraction. The new abstraction hole has supplanted the top three river holes (shortened the river) and it has lessened the flow in all the others.


It’s very simple: what goes in goes out. Under natural conditions it goes out down the river (plus the transpiration and evaporation I have mentioned). Under the unnatural conditions of an additional out-flow called “abstraction” the flow in the river diminishes: in this case by the exact amount abstracted, in the real world by an amount that is proportional to but not quite the exact amount abstracted (because of the other forms of outflow).


It stands to reason therefore that if we stop abstracting – or in this case put a bung in the “abstraction” hole in the bucket – the aquifer level will rebound and the river will eventually recover to the same level it was at before the abstraction. This is called “flow recovery” and it is the key idea behind Chalk-Streams First. 

Detailed modelling of flow recovery in chalk-streams in Dorset (the River Tarrant) and Berkshire (the Kennet) – both slope-face streams similar to the Chilterns rivers – suggests that for every unit not abstracted from the groundwater in the upper valley, approximately 80 to 85% of that unit will become surface flow in the river. 

So …. Let’s stop taking water from the aquifer. Let’s allow it to flow down the chalk-streams. Then let’s take it from the lower end of the catchment instead, after the chalk-streams (and the fish, birds, plants and insects) have had use of it first.

Hence we have called the scheme Chalk-Streams First.

Chalk-Streams First very simply makes use of the way chalk-streams function by moving the point of abstraction from the groundwater at the top of the valley, to the surface water at the bottom of the catchment where it can be taken into storage in the big reservoirs around London.

The obvious question which follows this simple idea is, how do we provide water to those towns formerly supplied by the groundwater, when all the water is now downhill at the bottom of the Rivers Colne and Lea?

The answer is a pipeline scheme called “SUPPLY 2040” which is already included in Affinity Water’s business plan. Affinity Water plans to build this pipeline (in fact a development and reinforcement of existing infrastructure with additional components and sections) anyway, to move water from their own excess zone south of the Thames to the deficit zone in the north. It is also needed for many other strategic infrastructure schemes currently under consideration, including Abingdon Reservoir and other options.

SUPPLY 2040 would enable the water that has been liberated to flow down the chalk-streams (or its equivalent volume) back up to the towns currently supplied directly from the groundwater. Better still SUPPLY 2040 could relatively easily be shifted forward to become SUPPLY 2030, meaning the re-naturalisation of all the Chilterns chalk-streams is within reach in less than ten years.

What we need now is a really detailed, independent investigation of the viability of the scheme. The Chalk-Streams First coalition has asked RAPID to run that investigation (RAPID has been set up by OFWAT to administer the strategic review of water resources). So far, the reception of the idea has been really encouraging.

But the more this scheme is talked about, the better. We need it out there in the conversation. If Chalk-Streams First can work in the Chilterns it could eventually become a model for how we save other chalk-streams in the future.

It’s high time we put Chalk-Streams First.


The evolving channel

A series of photos showing the new channel evolving from first cut of the turf last September up to this April. What the pictures don’t quite show is all the patches of weed starting to take hold and the sheer numbers of fish that have moved in. The next step will be roughing up the channel with the timber we have laid out along the banks,  creating high-energy pinch-points, undercuts and so on, aiming for a high ratio of bank-length to linear river-length, and also structure and cover while we wait for the bankside grasses and river weeds to develop and contribute to that process too. I’ll post more pictures as the channel develops over the next few months.



Building a river: lessons from Phase 1.

Having just finished the first phase of our WEG-funded project, I’m taking stock of how it unfolded, to better inform the next, much bigger phase of works scheduled for next year. I divided this 1600 metre re-meandering project into two partly because it felt like too much to deliver in one go and partly because I felt we would learn a lot from doing a short phase first, with time to gather thoughts over the winter.

It was touch and go in the summer whether we would get started at all in 2019, but I’m glad we did because we learned a lot.

Ground conditions were a challenge: we were working on a peaty floodplain, where the gravel was up to 80 cm below the surface and the groundwater half that. That kind of ground can take very little traffic before it breaks up. The site is a SSSI, so keeping it in good condition was paramount.

Groundwater was also a challenge. Although we cut ‘in the dry’, the channel would fill with groundwater overnight: even in late September, with groundwater levels at their annual low-point.

In some ways it might have been better to start at the downstream end, in order to cut the channel in such a way that it drained as we carried on upstream. But that would have presented another set of problems, not least that (for now) we were returning the new channel into an impounded and raised section of old channel: this backed up water level would have flooded back up the new channel and given us even more of a water ingress headache.

So we started at the upstream end. For the first week the weather remained bright and the dryish ground held up very well … so long as the tracked dumper took straight lines back and forth with the spoil. This first section of spoil was placed along the edge of the existing river channel, ready to be pushed in at the end when we made the cut through.

But then as we worked downstream we came into a more fragile, peaty part of the floodplain. In addition from a certain point on we had to start taking the spoil off site and this across and off the floodplain.

We worked out a methodology so that the digger always worked within the confines of the channel it was cutting. This meant that we could keep the surrounding ground intact, but it also meant we got just the one pass at cutting the channel to the exactly correct levels for the pools and riffles.

We brought the dumper up the line of the new as yet undug channel and filled it by turning the digger a slightly laborious 180 degrees, one bucket at a time.


This worked okay for about a day or two. But where the dumper tried to follow the meandering course of the as yet undug channel it began to cut the ground in the tighter turns: tracked vehicles turn by going faster on one side than the other which creates a shearing effect on the ground. Eventually the floodplain surface broke apart and after that the dumper started to churn the ground up , to the point that it almost got stuck on a few occasions.

So we decided to take the peat away first and excavate down to the hard gravel which we could use as a roadway. This meant the digger had to “hay-make” the peat into accumulating piles and roll it on out of the site to a spot where the dumper could run in straight lines to and from the floodplain.

This seemed to proceed reasonably, if long-windedly well, but groundwater would seep in up to the surface of the gravel, so what was exposed as a dry gravel road on the day we cut down to it, had become a soggy, muddy road by the following morning.

When we then cut through that gravel to the desired bed level, the channel immediately began to fill with water. Consequently we had to leave coffer dams of gravel when we started each morning to keep the overnight infill out of the new day’s dig.


And thus we proceeded, day by day, until the weather broke and things got really tricky. Once it started to rain the ground because even more hazardous. After one, long and very wet weekend we returned to find our new channel absolutely brimful, like an infinity pool.

We tried pumping the water out, but this was a hiding to nothing really. The pump kept blocking, or sucking in air and stopping altogether. Somehow, by hook and by crook we managed to get to the bottom end and to hay-make the material back out to the one spot the dumper could reach without having to turn.

This main drag to and from the river became a real mess, but it was, at least, the only mess we made.

After all that the cut through was the simplest job of all. We cut a small channel from the existing river in to the new one, waited for the existing channel to drain down a bit and then carefully laid a large tree across the old channel. Building against this edge we created a land bridge across the old channel and then rolled on down from here filling in and levelling as we went.


On the way out we placed a goodly number of branches and ranks from trees that had been cleared a couple of years before. These will be pinned in place to add some diversity and grit to the new and still somewhat bare river channel.


I’m happy to report that trout, being curious creatures, didn’t take long to move in. Macrophytes, shards of starwort and ranunculus particularly, have already caught up on some of the stones. And I watched a kingfisher follow the new channel as we dug it.

I am now planning the fine details on the next phase.

If anyone would like to come and see the works do drop me a line. If a few people are interested we could arrange a field day and maybe a small workshop looking at some of the planning and delivery issues.




Breaking Ground



A few photos as we begin work on Phase 1 of our Water Environment Grant (WEG) funded project – see previous post.

I am working with the expert help of Five Rivers, Stew on the digger and Jari (aka Gary) pedalling the tracked dumper. I’m on supervisory duties with the laser level and a satchel of drawings.

We have a brand new wide-track (90cm) excavator to play with. This machine has unbelievably low ground pressure: “like riding around on bog mats,” says Stew. This is good news as the ground here is very delicate and relatively saturated even at this time of year. The new channel fills with groundwater overnight.

So far so good: the peat floodplain is holding up, so long as we run the machines in straight lines, while the gravel is down there pretty much at the depths my unscientific road-pin probing survey indicated.

The real-challenge is trying to recreate the distinctive shape of a natural, spring-fed pastoral river – those undercut banks and the terraces on the insides of meanders, the gentle rise and fall of the river bed, the beautiful and almost inimitable randomness of a natural river  – while every other part of the process is about precise details and planning.


That’s where I hope my bursting-at-the-seams photo album of unmodified spring-fed streams in New Zealand will come in handy!

A new river

CAcreCommonApril2018 V1 copy 5

Three phases of a 1.75km re-wilding project – blue, light-blue and purple – in which the River Nar will be taken out of its diverted and impounded course and restored to a meandering channel in the centre of the floodplain.

In early September we will be starting work on the first phase of our most ambitious River Nar project to date, what will become almost 2 km of re-created “natural” chalk-stream: with funds from the Water Environment Grant we are moving the river out of a diverted, impounded and now dredged channel and putting it back where it belongs – in the bottom of the floodplain.

An awful lot of forensic work, sifting through old maps and aerial photos, of thinking, discussions, thinking again, drawing, measuring, and drawing again – not to say form filling and fund-raising – have lead to this. Probably rather more than ever preceded the diversion (several hundred years ago) and the dredging (twenty to fifty years ago). It takes more work to fix something than bust it.

Like most chalk-streams, the River Nar has been diverted along much of its course. The diversions have been driven by putting the river to use in one way or another over the centuries. It was widened and straightened in the early Middle Ages in order to float stone in barges to build the abbeys and castles along its course. It was diverted to feed water to those abbey kitchens, and latrines, and to power mills (some of which are recorded in the Domesday book). And in the late 18th / early 19th centuries it was used to create “floated” water-meadows, an agricultural innovation which trebled the crop of hay that could be taken off the floodplain meadows: the improved crop of hay fed greater numbers of sheep, which were “folded” at night onto arable fields on the surrounding hills, and thus fertilised those hills with the goodness of all that floodplain grass. Quite a neat form of cyclical land-use.

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An aerial photo from about 1946 showing the diverted channel (dark and straight) and feint remnants of the original channel to the north.

Whenever you take a river away from its original course and divert it towards and then along the edge of the valley, you inevitably lessen the gradient for the duration of that diversion, you turn a swift-flowing river into a linear pond (hence the word “impound”) and in so doing build up “a head” of potential water-power than can be released at key places, at a mill or across a water-meadow, for example. In a way these functional structures are all part of the rich history of the valley: but they have big consequences for wildlife. The Domesday mills shut salmon and sturgeon out of the River Nar over one thousand years ago, for example. And in more recent decades dredging and the sheer volume of silt that runs in to the river from surrounding farmland and roads, has greatly exacerbated the problems caused by diversions and impoundments.


A typical reach of diverted, impounded and dredged channel.

The diverted and impounded and now dredged and silt-filled channels are not nearly so good for fish, or invertebrates, or the plant-life one would normally associate with a free-flowing chalk-stream.

This project, therefore, is an attempt to “re-wild” the river, to give it back its gradient, meanders, deep pools and gravel shallows, and to improve conditions for the fish and the plants and insects which thrive in a natural, free-flowing chalk-stream habitat.

Castle Acre Common Phase 1 AUGUST 2019 jpeg for web

Outline plans of Phase 1.

We are going to cut a new channel – although in fact it’s our best guess at a re-creation of the original channel – which will meander back and forth along the bottom of the floodplain. The gradient that is currently expended over a very short part of the existing channel will be spaced evenly along the entire length of the new channel, creating a swift moving river that falls from one riffle and pool to the next.

We’ll be cutting the channel with diggers … but in fact cutting a new channel is something natural rivers do all the time: even spring-fed rivers such as the River Nar. Have a look at the aerial image (below) of a spring-fed river in New Zealand, one that has never been straightened or modified and where the floodplain either side is open, uncultivated grassland. There are relic channels and ox-bows all over the place. In places the river runs through several channels side by side.

Screenshot 2019-08-26 at 12.48.23

The River Nar is doing the same thing in one section of our new project site: a breach in the impounded leat is allowing some of the flow to escape through a willow carr and thus find its way back to the remains of the original channel. If we left it alone for a thousand years, it would probably finish the job on its own. This project puts those natural self-repair processes on fast forward.

In planning a project like this, an awful lot of work goes into investigating the history of the river and how it got to be how it is. Old maps and aerial photos suggest that the river here was diverted first to the north and then to the south to create a crude water-meadow or perhaps simply to shift it out of the way of the floodplain meadows.


There is evidence of basic catch-work drains along the pathway of what would have been the original channel, and of smaller drains running perpendicular to the original river. Towards the lower end of the project site the existing channel is suspended higher and higher above the floodplain, held behind a leat-like earth wall, until suddenly the river passes a sill in the river bed and over about 300 yards all the accumulated gradient is released in a headlong rush towards a mill. It is the strangest mill-leat I’ve ever seen, but maybe it worked.

Luckily some of what is almost certainly the original river channel remains in the valley floor towards the lower end of the project site: this grown-in channel meanders along an old parish boundary.

The first phase – 400 meters – will be cut this September. We will cut the channel by digging down through the floodplain peat until we get to gravel. But the heights of each riffle have to very carefully worked out in order to ensure we build a river that has a gravel base and flows downhill! You can’t just excavate until you hit gravel: you have to excavate to a very specific set of heights and try to coincide this with the gravel that is down there. Since you also don’t want to be digging and removing tons of gravel, you have to run the course along a pathway that coincides with where the gravel is at the correct depth under the floodplain. There’s no easy way to do this: one day someone will invent a cheap underground radar. In the meanwhile it’s me walking back and forth with a Trimble levelling device and a metal road-pin which I drive into the ground until I hit gravel. I then try to build a map of the floodplain surface and subsurface, the heights of the gravel under the surface, in order to arrive at the best course for the new river, trying to coincide this with a best estimate of where that original course would have been. It’s easier when there is an old channel, or evidence of one, to follow.

The peat that comes out of the ground will be used either to repair any compression of the floodplain caused by the machinery or it will be taken off the floodplain and used to fill an old gravel pit near the site. Some of the spoil will be used to plug the existing channel at the cut off point downstream as far as some freshwater springs: these will continue to feed water to the existing channel which will then become a fen-like oxbow channel and add diversity to the range of habitats in the valley.

I will post pictures as the project unfolds. In the meanwhile here are some before and after mock-ups of what I hope it will look like:

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The difference five years makes.


The photograph above of the River Nar near Castle Acre was taken in 2015, just after we had strimmed the banks ahead of rehabilitation works. The issues here were a dredged channel and a steeply domed riverbank piled high with the arisings. The channel is too wide and too deep. There is little hydrological connectivity except on the far bank where a berm has formed within the overwide channel. The ‘dredging’ which was more the result of mechanised weed-management than a concerted dredging effort had taken place annually, and always from the same bank. The programme was haphazard too, in that no clearing was done in inaccessible parts of the river: in the far distance a line of trees had protected the river and here the bed is undamaged. In an ideal world we would have built up the river bed with gravel before doing any other work, but gravel was not easily available here and our budget precluded us from bringing any in. We did fill two short sections in the reach as photographed. Otherwise the work consisted of installing LWD and then re-grading the bank to create a narrow, sinuous channel and a much lower bank profile which the river can – and does – spill over in high flows. The progression of photos below shows the evolution of the work over the following years: 2016, 2017 and 2019.


Newton Common – a new bed to lie on

Now that the project is two years old I thought I’d share a few before and after shots of the work. Newton Common is the next reach of river downstream of Emmanuel’s Common. You can see from the before photos that river here had been canalised and deepened and was as a result pretty much overrun with bur-reed.

I made a long-section survey of the river-bed which revealed the places where the bed had been lowered below the natural fall-line of the river. Each of these was effectively a sump. The resultant siltation was the river’s attempt over time to repair itself: we gave the stream a helping hand with as complete an infill of gravel as we could manage within the budget and the time.

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A bed-level survey revealed the sump-like reaches where the river had been lowered beneath the fall line of the valley. The Newton Common project is marked by points 1 to 29.

NarNewtonC'n + Hall Meadow 18:11:16





The top image was taken in October 2016: the channel is covered with burr reed. The second image was taken before the project began and shows how deep and wide the dredged channel was. The third image was taken just after the gravel went in, April 2016 and the fourth was taken two years later in April 2019




The top image was taken in October 2016, the middle image during the project and the bottom image in April 2019: note the presence of ranunculus and berula!

Notes on Large Woody Debris

_DSF5368Learn from nature: LWD is rarely better than when an old tree falls in.

Large Woody Debris (or in plain English ‘Trees in Rivers’) – if done well – is a very effective river restoration / rehabilitation technique.

Until very recently we tended to tidy rivers up. In fact we still do. I often see pictures posted on Instagram and Twitter of tidying up exercises on chalk-streams masquerading as river restoration: “clearing out” a stream, or uniform and tidy constructions – faggot hurdles, bank edges – that would be more at home in a garden or golf course. 

Traditionally river-keepers tended to pull out trees and branches that had fallen into their streams: the aesthetic was one of a ‘tidy’ river, the practical intention was to make the river easier to fish, with fewer obstructions to casting, and no tangly obstructions to lose fish in. It was about creating an environment that suited anglers more than it suited the fish they were seeking.

State bodies like the Environment Agency have colluded with this mindset. When a tree falls in they remove it, motivated by the need to avoid ‘flood-risk’.

But slowly rivers-keepers and the Agency are changing how they look at this issue. That’s because in both fishing and habitat terms and in terms of flood risk, removing trees and large branches from rivers is self-defeating, the very opposite of what we should be doing.

IMG_1698Trout love cover: remove the cover, remove the trout.

From the fish and habitat point of view it’s simple: fish love cover. A tree in a river provides fabulous refuge from predation. In New Zealand’s spring creeks which are not tidied up at all, the largest trout will always lie within bolting distance of a submerged tree, or some form of log-jam or snaggy overhead cover. This makes catching them tricky, but on the other hand they wouldn’t be there if the LWD wasn’t there too.

Research carried out by Dr Murray Thompson has also shown that LWD is also fabulous habitat for invertebrates, that invert numbers are far higher in rivers where there is plenty of submerged LWD.

LWD is also great from a morphological point of view, especially in chalk-streams.

Chalk-streams are very low energy rivers. The forces which shaped them (melting glaciers) have long since retreated from our landscape. We have since that time heavily modified their channels by straightening, dredging and impounding them. Chalk-streams very rarely achieve the flows needed to overcome these modifications and re-shape themselves to a more natural planform. Thus their shape and their habitat is locked in a man-made straight jacket and the rivers are effectively imprisoned.

But a fallen tree can make a huge difference, releasing gravel and stones into a system that lacks material (quick morphology lesson: when a river is modified, by dredging, say, the natural processes of the river work to erase that modification, depositing material into the enlarged space until the river reaches the correct dimensions again. To repair itself, therefore, a river needs material, but chalk-streams are too gentle to supply it, except in the form of fine particles like silt. When a tree falls in, however, this creates the material and energy that enables self repair). A single tree can kick-start a whole sequence of channel repair and re-meandering.

I own a fishery at Frampton in Dorset where the river once flowed through a stately park, impounded by ornamental weirs. All the bends had been taken out. When I bought the fishery the then owner removed fallen trees all the time. He even removed one “for me” after he had sold me the river. I had to explain that I really didn’t want him to.

DSCF5328One large filled tree is visible in the distance, the other to the left of the shot. Ten years ago this reach was dead straight. The entire green bank on the left is new and the bend to the right is new. The same has happened in the opposite direction downstream of the lower tree.

DSCF5610Looking upstream from the lower tree-fall. The vegetated island in the middle of the shot and the meander to the left were created by a single fallen tree upstream.

_DSF43882013 when the lower tree fell in.

DSCF53272014, one year later: we’re starting to see a braided channel, a deep pool, a meander.

DSCF6881The third big fallen tree lies upstream of the island and gravel riffles which it created.

Since then three really big willows have fallen in. I have left them where they fell and watched what happened. Those willows have catalysed a whole sequence of channel repair, far better than any river restoration guru could ever have designed. The Frome is quite a powerful chalk-stream, so these changes have been on fast forward compared to gentler rivers, but even so they are extraordinary: hundreds of yards of re-meandered stream, of pool and riffle sequence, all catalysed by three fallen trees.

Those willows and many other instances where I have observed the aftermath of a natural tree fall, suggest to me that even now with LWD “restoration” we choreograph our work far too carefully. I’ve done it myself. In fact on the 7km LWD project I did with Simon Cain on the river Nar the work got more cost-effective, more natural and far, far more efficient as we progressed. To me good LWD imitates the impact of a fallen tree: it begins and ends with that.

_DSF4010Early phase of LWD work on the Nar: this was pretty good. It worked well but was quite intensive.

DSCF2512This was better! Less intensive, more natural, more cost-effective.

Another important learning curve has been where and when to use LWD. Basically there is a depth and gradient beyond which LWD doesn’t really work very well, at least from a morphological point of view.

LWD makes it best impact in relatively shallow, wide reaches, where the bed is more or less intact and where the river is not impounded. It is perfect for repairing an overwide river, but is not nearly so good at repairing a dredged river or an impounded river. In the latter two cases you need to resolve the river-bed and gradient issues first.

Do’s and Dont’s of LWD in chalk streams

• Don’t overly choreograph your LWD. Resist the desire to “build” something tidy or uniform.

• Look at and learn from natural tree fall and try to copy it.

• Use big timber, whole trees or very big branches.

• Work with the natural meander pattern of the river. Work out what the natural meander wavelength should be (see my other notes on this) and use LWD in sync with that, on the insides of bends or what will become bends.

• Think about daylight: the LWD will allow sediment to accrete in the slackened flow downstream. This sediment will only consolidate with vegetation and vegetation needs daylight.

• But don’t overdo the above. Rivers need shade too. Try to create a mosaic of light and shade.

• Be judicious about which trees you cut down. Alder is best. Willow can be great but is difficult to control later. Ideally you will have multi-stand alders and you can use some and leave others.

• Don’t expect LWD to do much in an impounded or overly deep river. Really you should remove the impoundment (weir or mill hatches) first. When you do you will have a fast flowing but overly wide and homogenous channel. That’s when LWD can work it’s magic.

Murray Creek. West Coast.Natural LWD in a New Zealand spring creek: spot the trout within a fin-flick of tangled cover. Note in the pictures below how the really big LWD has shaped the channel. It’s like this the whole way up the river!