Change in sediment regime (rivers)

A change in the nature of the dominant sediment processes that occur in a river. The sediment regime is controlled by a range of factors such as grain size, sediment supply and prevailing flow conditions, and can change in response to any variations in these factors. Rivers can act to transport, supply (through erosion) or store (through deposition) sediment. All of these processes might be expected to occur throughout the river network with headwaters tending to supply most sediment and lower reaches to storing and export sediment in general terms. Accelerated processes of erosion or deposition are a sign of problems with the flow regime or an over or undersupply of sediment through the system.

Impacts and causes

A change in the sediment regime of a river channel can cause significant changes in the prevailing geomorphological conditions of the river system. Change can be continuous with gradual recovery or degradation of the sediment regime but in some river systems river thresholds in behaviour can be crossed where the regime switches, for example, if sediment supply reduces to a point where the river becomes 'starved' of sediment, an erosive regime may develop which can result in increasing in channel size and lateral migration or both. Conversely a change from an erosion or transport dominated regime to a deposition dominated regime can cause a major geomorphological response, with new in-channel sediment forms developing. The geomorphological responses to a change in sediment regime can lead to a major change in the types of habitats that can be supported by the river. For example, species which require stable conditions are likely to prefer a depositional regime, and may not survive a shirt towards a more erosional environment.

The following flood risk management and land drainage activities are likely to cause a change in the sediment regime of a river:

Mitigation measures