Wednesday, 9 January 2013

Small Stream Adventures — The Butterfly Effect

A few years ago I discovered caches of roach, perch and pike in my local River Sowe. It's a very small stream where I live in Longford but quite large enough to support fish populations along its entire length from Bedworth all the way to its confluence with the Warwickshire Avon at Stoneleigh. The trouble is the river has been extensively altered over the past century and that has resulted in a progressive denuding of possible habitats to the point where fish actually can live in very few places today.

There's a few areas that remained untouched and are almost exactly the same river seen on Victorian maps with the various meanders unaltered in the intervening years. Those ancient and natural stretches are where the fish are, they exist there in healthy numbers, and are doing rather well with both roach and perch growing to a healthy peak weight of perhaps a pound and a quarter. These are shoals of fish that have lived in the river since the last Ice Age.

Above and below that narrow and winding natural stream of a 3/4 mile length there's a straightened trapezoid channel that runs in both directions for miles before suitable habitats are found once more. Below the stretch I once walked Molly my springer spaniel, who's an invaluable companion when searching for fish, and together we explored and examined the river closely all the way down to the confluence with the Smyte Brook near Binley. She failed to spring a single fish from cover along the entire three miles, because there wasn't any cover to speak of. Fish only require enough water over their heads for security and they'll thrive, but along the entire length the channel was no deeper in any place than two feet at most and the majority was less than a foot. It was unnaturally wide, unnaturally shallow and unnaturally empty.

Above the natural stretch the story was similar but not quite so depressing. There's almost a mile of the same manmade 'flood relief' channel running from below a recently constructed weir made of large red sandstone blocks in Longford Park. Again, no deeper than a couple of feet and mostly far, far less than that. Above the weir are potential habitats once again — they too are empty except for bullheads, loach and sticklebacks — but at least they are there! The only trouble is the weir is an insurmountable barrier.

So there's an impasse between the reaches a mile downstream where healthy populations do exist and where they might live happily caused by a too shallow channel that fish of size cannot live within, dare not venture up, and if they ever do, will find no cover and so will have to come straight back down again to a safe environment. This means that the populations in the natural stretch are trapped where they are and cannot expand into new territories. They're fossilised.

That was true last year as it had been true for nigh on a century past. But this morning I discovered an astonishing thing — roach had made it up to just below the weir and were now living there!

It's a small miracle they have managed to colonise the place but not quite an accident or plan of nature because I seem to have created the conditions for them myself. Just below the weir there's a fallen willow sapling that last summer was trailing branches in the water half way across the stream and I noticed this had caused a small change in the riverbed. Enough of the flow had diverted around the obstruction to scour out a slightly deeper channel opposite and a small pool had formed behind.

It wasn't a great change, certainly not enough of a change to create a fish environment because the obstruction wasn't solid enough to achieve that with much of the water still passing through the trailing branches, but I thought I'd have a little dabble in the way of amateur 'river management' to improve things. I went up to the weir and brought back a few heavy slabs of concrete, large stones and bricks. These I placed under the willow branches to create a small groyne running at a diagonal half way across the stream.  Handfuls of smaller stones were piled over it to hopefully plug the gaps and then I left it alone to see what, if anything, would happen.

After the next decent fall of rain had run through I went back to take a look. The heavy stones were still in place, woody debris had accumulated and blocked it up nicely, some of the small stones had been swept away but it was now a quite solid obstruction. What was interesting though was the effect it had had. The channel to one side of the groin had not only deepened by four inches but had scoured all the way back the red clay, the pool behind was now larger and deeper than it had been and the scoured stones had swept down and created a shallow riffle downstream. A baby meander had formed.

This morning I thought I'd take another look. After the torrential and continual heavy flows of recent months I expected the entire thing to have been swept away along with the willow sapling, but not only was it still in place, it had grown into something far larger and more impressive than I'd ever have imagined it could. A great raft of woody debris had accumulated against the willow and the water simply flowed around now it as if it was solid as rock.

The effect was magnificent. Everything was now larger and more substantial — for twenty yards below the groyne a deep pool had formed, then a shallow riffle and another even longer stretch of deep water had formed below them. Still far too wide for its own good but was now looking something like a very short but very natural stretch of river. I sent Molly in (... and Oscar too, her 6 month old pup who's in water training right now!) to see how deep it really was in these pools because if she had to actually swim then the little pool of last year had deepened and expanded considerably downstream since last year when the water would have come up to her chest, at most.

She launched herself after a missile chucked under cover at the far bank, and sank! It was well over two feet deep and it was over well two feet deep for many tens of yards downstream... The missile sank too, and that's the trick that keeps her at it. She'll swim about and dive down over and over till she finds it on the bottom, which is a funny sight — a diving dog, but an asset with this kind of 'work.'

Anyways, she'd got herself caught up in a mostly submerged shopping trolly (which means 'deep enough' in of itself) and a tangle of the roots and branches of a small shrub and whilst extracting herself sprang a large fish —

Huh? That's a... a..... A ROACH!

— who glided calmly across the surface before vanishing into the murk Molly caused in her hunt for the missing missile.

I was amazed and confounded. Elated too. How the hell had a roach got itself up here in the first place? There were none here ever before, believe me —I'd have seen them, but now there were proper fish living out the back end of my experimental groyne.

I supposed they'd got there during the floods when the sheer volume of water then would have afforded safe passage all the way up the otherwise barren mile? I didn't know fish did this — but they must. How else could quite large fish, and being roach there'll be a shoal of them without any doubt whatsoever, suddenly be found living in such a place otherwise, unless this is how they do it? Perhaps they've been trying for a century to expand into new territory here during every single flood but have been thwarted over and over again? I don't know.

What I do know is that rearranging the river furniture had had far reaching effects over a short stretch of river that may well have created a viable environment for them. We'll see. When the water falls too far they may be forced back downstream once again. Nevertheless, it would have been worth the small effort for such a great result if they stay for just for a while. I do hope they stay forever though — that would be quite something, don't you think?

Ooh. New hobby! I won't stop here, you know. I'll try again and again, till some kind of river is created where no kind of river was before. Bloody hell, what fun — now I'm an amateur river restoration freak!

Now, where can I find more fallen saplings to chuck rocks under...

... and how can I get those roach safe passage above that tricky rocky weir?


  1. Good work Jeff. Keep us posted.

  2. When I was a lad, I used to do something simiar to that in a small river which held trout, eels & brook lamprey. The features created would last a month at the most before someone would destroy them. Thets hope the council workmen don't undo yours while tidying up.

  3. They won't venture down there Martin, it's a twenty foot drop through a triangular manmade channel and a wilderness of rough scrub before you get to the river. Health and safety won't allow it without a special permit!