The regulation of streamflow can both positively and negatively affect existing fishery habitats, fish populations, and channel characteristics. This occurs because, in most geograpical areas, natural precipitation and run-off patterns have produced well-defined periods of low and high streamflow. The regulation of such flows can disrupt the biological communities which have adapted to the natural flow regime. In the western U.S., it was quickly recognized that uncontrolled development of water could result in the elimination of aquatic communities and dramatically alter channel morphology. This was alarming, since many of the systems in jeopardy harbored significant sport and commercial fishery resources, such as the salmon fisheries of the Pacific Northwest.