Enhancing spate rivers
Recently I was interested to see a photograph of two large diggers working in the River Tromie, a tributary of the Spey. They were fixing a large, dead tree, trunk and root-plate, on a gravel bank in the river, with the root-plate upstream. The purpose, apparently, is to cause the flow of the river to develop more natural features, such as meanders and gravel banks, to the (hoped-for) benefit of the entire riverine habitat.
This did rather make me smile as I well remember that, many years ago, the proprietors of a section of the River Findhorn were taken to task by SNH for using large diggers (working in the water), to make alterations to the flow of part of the river. This was, at least to some extent, to make it easier to fish.
What was being done then, was quite traditional. Fast rivers, especially short, West Highland rivers, often end up with lots of stony rapids, and few good pools; frequent spates move boulders and smaller stones downstream and the resultant, tumbling stony streams are not merely difficult to fish, but offer limited useful habitat to the fish themselves. I have no idea when men began to do small-scale improvements to such rivers; it could easily have been around the time that people began to construct stone fish-traps or cairidhs in sea-lochs like Loch Torridon (and I have no idea when that was either!), but, as interesting, small-scale interactions between our species and nature, they should take their place alongside the creation of machair crofts, hay-meadows, and the traditional management of native woods.
There are times when successive spates can cause the formation of shingle/stone ridges right across the flow of the river, and at times of low water, this can create a band of such shallow water that migratory fish, like seatrout or brown trout wanting to migrate up the river to spawn, are trapped below it until the water level rises. By excavating a narrow channel through such a band of stones, and using the material from the new gap, and other stones or boulders to enhance the remaining level on either side, small piers or ‘croys’ are created from either bank. This will often in itself raise the level of water upstream, creating a better pool above, while allowing easier access up the river for any waiting fish.
Such croys were often constructed at the foot of any pool, in order to raise the water-level and to create a good central flow, as described above. I have also seen very large square stones placed (like stepping-stones) in a curve across the foot of the pool, which has approximately the same effect, although, of course, it does not hold back as much water.
As to the benefits of creating such a pool-and-croys system, any fisher will tell you that fish do like pools; it is in such places that they will grow to a decent size, particularly if the pool is overhung with birch, willow and other trees. And the calmer flows of water permit the growth of a number of aquatic weeds, where caddis-flies, for instance, may easily be found. Meanwhile, the calm back- and side-waters are excellent nursery-places for what we always called “tiddlers” when we were young. And dragon- and damselflies love such peaceful waters….
Many of these rivers have lost some of the gravel banks where fish choose to spawn; constant spates have washed out the smaller stones. It occurs to me that it would be feasible to create such gravel-based shallows just downstream from one of the angled croys. In the comparative ‘shelter’ of the croy, the gravel might possibly stay longer in situ, and, in any case, it could quite easily be replenished from the bank or the croy itself.
On bigger rivers, the croys are often wider and flat-topped, and so used as stances for fishers. The River Inver in Assynt has a very large pool, the Star Pool, held back by quite a substantial dam, upstream from which there are several pairs of croys, normally used in this way. But the calm, comparatively shallow water towards the banks is still inhabited by tiddlers, while water-birds like dippers and grey wagtails take advantage of the varied habitat so created.
As it appears that NatureScot now approves of anchoring tree-trunks in riverbeds in order to make changes in their flow, (assuming the first real spates do not send them further downstream) I do very much hope that the renovation of old croys or the creation of new ones will equally be approved. Small human interactions with nature can very clearly enhance the biodiversity which we all know we need. Croys-and-pool systems do exactly that.