Water obviously, but also wood and sediment (gravel). If any of these 3 things are missing then a river can become unhealthy very quickly.
Rivers, streams and brooks are dynamic. The processes of erosion, deposition, and siltation mean rivers are constantly changing. This constant change creates many of the specific habitat requirements needed by river wildlife.
As an example Brown trout need:
– Deep pools to hide from predators and hot weather
– Gravel riffles to lay their eggs
– Eddies to rest up and feed
– Backwaters to lie low in during large spates and floods.
If these habitats are missing from a river, so are the trout.
Water is the driving force of these processes, each taking place at different flow levels. High flows to carve out pools but also low flows to deposit riffles. A healthy river has a mixture of these flows across the channel, some areas flowing quickly others slowly.
Wood plays a key role in creating this variety of flows across the channel. When a piece of wood falls into a river, it diverts the flow, scouring the river bed and creating pools and riffles.
This river lacks wood, that’s why the flows and habitat are so uniform (like below).
Gravel and sediment provide the material from which these habitats can be created, without gravel or sediment, you don’t get riffles or deposition. You only get a deep river with bedrock, which suits only a limited number of species.
Lack of sediment is most obvious downstream of dams, which trap it all. The Wye and Usk Foundation undertook a project which moved gravel downstream of the Elan Valley dams. The result was a twenty-fold increase in Juvenile Salmon
https://afonyddcymru.org/restoring-ecology-in-the-afon-elan
Water, wood and gravel flow down a river, all three are needed to create a healthy river. Gravel and sediment provide the material for building diverse habitats, water to mould the gravel and wood to shape the water flow.
To ensure a healthy river, these natural processes need to be taking place, without them, we will not have healthy rivers.