This has the affect of greatly reducing flow variability downstream by capturing smaller frequent floods. Larger infrequent hazardous floods are much less affected as they tend to fill the reservoir and then spill uncontrolled at the flood peak These changes in flow variation and flood frequency have major adverse impacts on key processes. Recently attempts have been made to release small flood pulses from some reservoirs to mitigate these adverse affects. Such measures alone are unlikely to be more than marginally useful. If they do not also restore the flow of sediment to the river.
Large reservoirs have the capability of capturing almost all of the sediment carried by a river.
The impacts on river system habitats are profound. Lowering the channel bed isolates the floodplain from ecologic interaction with the river channel as effectively as if it were leveed. and channel lowering lowers the watertable drying up floodplain wetlands and woodlands. Further downstream muds no longer replenish estuaries and deltas, sands are no longer delivered to beaches, degrading estuarine and coastal habitats. In the U.S. we know that technical fixes like fish hatcheries are not working very well, but we also see attempts to restore landscapes without restoring physical processes. Our conceptual model of a living river leads us to a meaningful approach to restoration.