Fish, bugs, amphibians, riparian plants-all depend on a river's seasonal cues for reproduction and other life cycle processes. These are processes that have gone on for eons. A river altered and harnessed for hydropower no longer produces these biological cues, and as a consequence fish and wildlife frequently suffer population declines. Most hydro projects, at certain times, divert a majority of water out of river channels and into canals and pipes. Another top priority is to restore nutrient richness and provide for healthy spawning habitat in streams by improving the passage of sediment, gravel, and the natural debris of branches and logs.
Fish depend on a specific size of gravel for spawning.
They also need logs, brush and plants to create important shade and refuge from predators. Dams often block sediment and woody debris from flowing downstream, and provide habitat for the aquatic insects they feed on which depends the natural regeneration of ecosystems within a river channel. And it is not only fish that need healthy rivers. Vegetation along the streambanks also need those nutrients created by the movement of sediment and the breakdown of woody organic matter. Without healthy riparian plants, streambanks erode, destroying the shaded cover and widening river channels.