A Historical Analysis of Surface Water Storage Development and River Bifurcation in the United States

Abstract:

The number of free-flowing rivers in the continental United States (CONUS) has consistently declined as a result of dam and reservoir construction over the course of the last 300 years. Today, more than 50,000 structures impair flow across every major watershed in the US. These anthropogenic structures bifurcate river networks, threatening the natural exchange of water, sediment, and organisms. However, dams and reservoirs are also beneficial to human systems, mitigating flooding, producing energy, providing recreation, and supplying water. Water managers and stakeholders require analysis of the current state of river bifurcation to weigh its importance and better inform water management practices that involve dams. This study used three indices to evaluate the degree of river connectivity in CONUS: degree of fragmentation (DOF), degree of regulation (DOR), and consumptive water use (USE). DOF was calculated from estimates of the natural average discharge of the river reach and at each dam, DOR from estimates of upstream storage along a river reach and the annual, natural average discharge volume, and USE from estimates of the natural discharge without anthropogenic effects and the average discharge after anthropogenic abstraction and use. Each of these estimates were contained in the river flowline or dam attributes of the National Hydrography Dataset and the National Anthropogenic Barrier Dataset. Using these values, we map and evaluate the degree of river connectivity across every major U.S. river basin. We consider both the current system as well as how fragmentation has occurred over time, and the major drivers of fragmentation based on reservoir uses.