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.

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. 

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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. 

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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”.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Breaking Ground

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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!