Trees are a key component of a healthy river and an important factor in creating resilience in the ecosystem to floods, droughts and pollution. They are important for rivers because:Trees provide shelter for aquatic animals. For example, mayflies need trees to rest and to metamorphose into the final breeding stage of their lifecycle; trout use overhanging branches and tree roots to hide from predators. Trees provide leaves and woody material to the river – this is a primary input of nutrients and food for detritus shredding organisms such as Gammarus, which are important food for trout and birds such as dippers. Trees provide shade during periods of low flows and high temperature, reducing water temperatures and helping to maintain oxygen levels in the water. Too much shade can be a problem in suppressing growth of aquatic plants, but a mix of dappled light and shade is ideal. Fallen trees have a major role in creating a dynamic river. A fallen tree can trap sediment, create scour pools and clean gravels. Trees create an important buffer zone, reducing the amount of run-off that enters the river directly during periods of heavy rain. This run-off carries sediment and potentially also pollutants such. Tree roots stabilise river banks and can reduce the rate of bank erosion.


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