HOW HUMANS MADE OUR MODERN RIVERS

They’re far from the natural creations we like to assume they are... for centuries we’ve been controlling them to meet our needs, and the effects on fish life can be extreme

FOR MORE than 1,000 years humans have been modifying rivers. The Domesday Book – a national survey undertaken in 1086 – records many weirs, locks and shallow crossings on rivers all over the country.

Today this means that it’s impossible to find a river in the UK that hasn’t been subjected to modification of some sort.

Traditionally, rivers have been described as having several zones, based upon their fish-life, their steepness and features. Very often the top of the headwaters are devoid of fish, owing to their steepness and the presence of natural barriers, such as waterfalls.

Within a short distance, though, fish start to appear, normally brown trout, which can withstand the tumultuous conditions. Here the river has a rocky bed.

The next zone begins as the river widens and the gradient eases. Here grayling are common, the cool water suiting them and the clean stony bottom ideal for spawning. Minnows will also start to be found, along with the occasional bullhead. Trees will begin to line the banks, although there will only be limited plant-life in the river itself.

With the river perhaps 10 metres wide and the gradient slackening even more, the coarse fish start to appear. This is the barbel zone, although chub are likely to be more numerous, along with dace and smaller species.

Thick beds of aquatic plants are likely to be numerous, and the water will be significantly warmer than higher up, giving rise to a wider range of insect species, including mayflies.

The final zone has a sluggish pace with the flow hardly perceptible in summer. Here bream thrive, along with roach and many of our lowland river coarse fish. Known as the bream zone, the river here will be wide, with a silty bottom interspersed with gravel bars and rich weedy margins that harbour huge swarms of Daphnia and other micro invertebrates, along with coarse fish fry.

Often the river will be lined with shallow pools that are refreshed when the river floods. This is the natural habitat of carp, tench and crucians that are well adapted to live in this warm, oxygen-poor environment.

Obviously, a natural river

contains few obstructions, such as weirs, that not only modify the habitat, but which block the upstream movement of fish.

Only recently have we come to understand how far coarse fish can move. Weir building can stop fish completing their upstream migrations to spawn. This annual upstream movement of the adult fish to reproduce balances the downstream drift of the tiny fry, which are thought to travel equally long distances downriver. Remove the first part of the equation and it is obvious that if the adults can spawn at all, their progeny will end up being washed progressively downriver.

Weirs also massively change the flow and topography of a river.

Most weirs were installed to raise the water level above them, to provide a head of water to power a watermill or to allow boats to travel over shallow reaches. However, increasing the depth has a consequence. The water slows, silt being carried by the river is dropped and so the upstream side becomes muddier, loses shallow gravel riffles and tends to choke with plants. The river can change from being in the grayling or barbel zone to the bream zone, an effect that can be felt several kilometres above a weir.

Downstream the changes can be just as dramatic. Speeding up the water flow and dropping the level immediately below the weir can wash out tons of material and deposit it further downstream. Suddenly the habitat is more suitable for species from the barbel and grayling zones than the bream zone, although this doesn’t last for very long.

Human intervention has also massively changed the habitat alongside many rivers. Most rivers cannot now spread out on to the flood plain except in times of exceptionally high water. This was not always so, and often the river and its flood plain would gently slope into one another so, as the river level rose, its width would increase rapidly. This helped to reduce the speed and height of the river, and also gave rise to a wide range of habitats along the banks that were reliant upon the floods to replenish them.

The natural pools frequented by tench, carp and crucians are now gone, and our river fisheries are poorer for it.

Higher up the river, the sideways spread is less, but here the floods have other effects.

The sheer power of tons of water per second flowing downstream can shift even large boulders, so the bed of the river is in a constant state of flux. This movement keeps gravel beds clean of silt, providing the ideal spawning grounds for species such as trout, grayling, dace and barbel.

So the natural environment in our rivers has changed massively over a millennium, probably reducing the stability of our fish populations and leading to the rise of species that suit these simplified environments, such as roach. Along with a plethora of other influences that we have had on rivers, from local pollution to climate change, and it soon becomes apparent that the rivers that we perceive as being a natural environment, unchanged over the generations, is a very long way from the truth.

“Weirs massively change the flow and topography of a river”