Each carrier will have a point of equilibrium that is dependent on a combination of flow characteristics and bed slope and here lies the rub with many artificial carriers. Usually chalkstream carriers were constructed to feed water into either milling impoundments, many that are long gone, or as a high level “drowning” carrier for agricultural irrigation. As the name implies, carriers were constructed to take water from the main channel, usually located somewhere near the valley floor, and to feed it into a system that usually follows a contour that holds the water up above the level of the parallel main channel. By nature, these carriers often lack the natural gradient found in the main channel, although inevitably, many main river sections have often also been modified with water level control structures that were designed to push water sideways into the upstream network of water meadow carriers.
As far as optimum habitat for trout is concerned, this combination of impoundment of main channels and a network of flat-bedded carriers, often lacking any naturalistic flow régime, is the worst possible scenario for wild trout. There is a rather old-fashioned school of thought that suggests that if it wasn’t for man, then we wouldn’t have chalkstreams. Personally, I think this statement is cobblers. Yes, we wouldn’t have chalkstream channels that conjure up images of perhaps a classic middle Test beat but there would still be open flowing channels, albeit, heavily braided, shaded and not as weedy. Even wide, flat valleys have a low point and that’s where the flow would be for most of the time, although there would have been areas where the gradients are modest and when the river would have been a network of dynamic small streams percolating through wetlands. What’s not to like? I’ve absolutely no doubt that salmonids would have thrived in these primeval swamps, as some imagine them to have been pre-arrival of the miller, farmer and piscator. At least the keepers wouldn’t have had to mow the banks!
Now this article isn’t advocating any notion that it is either practical, or even desirable to turn wholly man-made systems into natural ones. However, common sense dictates that a creature like the trout, that for so long evolved in unmodified river systems, is going to benefit from at least some consideration of creating an environment that mimics some of those natural characteristics. A good starting point is to recognise that natural river-shaping principles apply just as much to man-made chalkstream channels as they do to upland spate rivers. River channels that are exposed to the natural rhythms of summer/winter flow variability will respond favourably with stable channel widths and although it is entirely likely that there will be areas of bank erosion and channel movement, this should be seen as an opportunity and not necessarily a threat.
To create a more natural environment into a man-made carrier will require that the flows entering the channel have more natural variability and this may require redesigning any offtake/feed structure. A square-sided slot just won’t work. For the carrier to function more naturally, the offtake structure should not have a bed invert or impoundment and must have either stepped, or tapered sides, so that as water levels rise in the main channel then the carrier will receive an increasing percentage of the available flow resource. An offtake that is designed to give a more naturalistic share of the available flow will be far more likely to settle down into a stable environment that doesn’t require constant intervention.
