Wednesday, 16 January 2013

Small Stream Adventures — Transformations

Now that water levels have fallen and the stream runs clear and bright I've been able to see what amazing alterations were made to the river during the floods that followed my experiment on the River Sowe.

Last autumn I'd found a little willow that was leaning into the stream and noticed that its trailing branches had made a small difference downstream. So, I piled a line of rocks, concrete slabs and bricks under it that reached a third of the way across at an angle of forty five degrees and watched progress.

Within days it was clear that the altered flow had cleaned the surrounding riverbed armour of its coating of dark matter and had begun to scour a few clean spots below. It didn't seem a great difference at the time but come the relentless floods of December I expected something more visible afterwards — either the groyne I'd made would have been swept away or it had stayed put and done its work.

It had survived and was an obstacle that'd grown considerably, the little tree having acquired a massive raft of debris. It had become a permanent feature of the river bank.

What was interesting was what such volumes of water had achieved. The groyne had made an enormous difference over fifty or sixty yards of what was before a very wide and shallow industrially engineered flood avoidance mechanism.

Just a foot deep for almost a mile downstream the channel was originally designed to allow excess water to shoot down unobstructed and that means that what we'll have is a slow flowing, silt bound and too regular channel that is pretty much sterile.

As it was. A regular trapezoid channel just a foot deep for nigh on a mile. Click for a better view

Now we had a dramatic change — a series of features that can and do support life and all were produced by one month of relentless rain...

As it is today after a month of heavy floods and one very small obstacle inserted beforehand...

You can imagine the heavy flow coursing around the developing obstacle and being an angler you'd know exactly what that would mean — a compression of water volume creating a high velocity flume that dissipates into the area below but in doing so creates vortexes. These turbulent areas are very powerful agents of change over very large distances...

In this instance just a small obstruction had caused massive alterations in riverbed topography carving out an extra two feet of depth below and then depositing the scoured gravel 20 yards downstream to create a very shallow riffle. But that wasn't all it achieved, because the first riffle proved to be the start of a new pool just as deep as the first but even longer and was itself terminated by another shallow riffle at the pool's terminus.

The pictures above had to be compressed in length and depth, the tree line is far higher because the river runs in a deep V shaped channel now and the pools are far longer than they illustrate, but they serve to show what occurred during the floods. What happened is remarkable – a series of life giving features that will persist even through drought because the riffles contain water at a strict depth just so long as the obstacle that created them lasts.

Believe me no damage was wrought by my changes. Except for silt loving summer weed it was utterly lifeless. Now, because of this small intervention of mine and the following floods that carved suitable habitats where none were before, by a miracle of nature roach now live in the pools, and I'll do my damnedest to make sure they not only stay, but thrive.

Here's a video that'll give you an idea of what has occurred. 

The video illustrates exactly what kind of channel the Sowe had prior to my experiment. It also shows what kinds of transformations can be achieved if a great deal of money is thrown at restoring a stream and the Hampshire Avon is just the kind of river that would attract high funding...

However, I now think that careful, well-judged interventions utilising only what lies about the river can achieve similar results but at zero cost apart from boot leather and elbow grease if an interested amateur is willing to make a small decision and wait for nature's periodic but essential floods to do the hard work and create great change...


  1. This post and the impending bad weather has prompted me towards a possible session a few miles downstream.

    Last year I saw a massive log going half way across the stream on essentially what was a long shallow run. Now you got me thinking it may of scoured out a hole somewhere downstream and that it might have a few virgin roach in residence.

    Nice one Jeff and good luck you up-streamer ;)

  2. Dan I reckon they'll occupy anything remotely habitable if they can get to Longford Park from a mile away during high water. You know the river, it's not good for fish in most stretches, is it? But it should be alive with them its entire length. Be interesting to hear what your log brings forth!

    1. Well if the snow comes down and I stick local I will get down in the bushes, give my log a good old squeeze and see what pops out...

      I am sorry Jeff! All this talk of logs, I could just not help myself. I am so childish I know.

      Will keep you informed ;)