This article from California Water Blog is a good primer on the environmental perspective—much of which is very valuable, as long as human needs remain the primary focus—concerning rivers and dams.
Dams are no friend to biodiversity. Once impounded, a river answers first and foremost to human needs, be it water supply, energy production or flood protection. Releases are measured and timed to satisfy these demands.
As a result, the river downstream loses much of its natural variability in timing, volume and spread of flows. Dams also block passage of sediment that scours the stream channel and deposits fresh cobble bars. These activities create and maintain habitats for multiple species, contributing to biodiversity.
But dams don’t have to be death knells of biodiversity. Operators can manipulate flows in ways that restore some of their ecological functions that promote diverse riverine animal, plant and fish communities.
Releasing flows for environmental purposes is not new. California has long required dam owners to release enough flow “at all times” to keep fish “in good condition.” Further, some water and power suppliers are required under the federal Endangered Species Act to release flows at biologically important times for imperiled native fish.
These “environmental flows,” as water managers call them, may help fish survive, but they do not necessarily create habitat that promote high biodiversity. For that you need to implement a suite of well-timed flow patterns that move sediment and can access floodplains and over-bank areas.
My research colleagues and I recently identified five types of flows that are key to creating multiple habitats. We presented them recently at the annual meeting of the American Geophysical Union in San Francisco.
We call these “functional flows,” as distinct from fish-saving “environmental” ones, because they provide certain geomorphic, ecological or biochemical functions that support breeding, migration, habitat diversity and, ultimately, biodiversity.
Retrieved February 2, 2015 from http://californiawaterblog.com/2015/02/01/how-dam-operators-can-breathe-more-life-into-rivers/