The Index of English Chalk Streams

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Photo Credit Roger Tabor Wikimedia Commons

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

A New All Party Parliamentary Group for Chalk-Streams

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

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

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

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

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

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

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

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

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

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

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

WEG Project Phase 2 – works in progress

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

CAcreCommon WEG Project Overview WEB

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


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

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

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

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

More thoughts on Stage Zero designs on chalk-streams


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

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

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

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

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

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

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

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

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

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

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

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

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



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

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


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



Stage Zero on an English Chalk-Stream?

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

Stage Zero – What is it?

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

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

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

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

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

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

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

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

A spring creek in New Zealand

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

Stage Zero on an English Chalk-Stream?

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

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

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

Large Woody Debris on the River Nar

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

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

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

An Accidental Stage Zero

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

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

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

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

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

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

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

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

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

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

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

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

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

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

A Domesday Legacy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.