Humans have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. We now recognize that the harnessing of rivers incurs great costs: commercially valuable species, native biodiversity, and ecosystem services are lost as a result of regulating the "wild" flow of rivers.

We argue that a wide range of hydrologic variability is critical to sustain ecological integrity of river ecosystems. The natural flow regime of an individual river determines its channel shape and the range of physical habitats supported, the riparian vegetation, and the biological communities occurring within the river and its valley. To best conserve and restore river ecosystems, a principal management objective should be to restore the river's flow as closely as possible to its natural pattern of variability.