Rivers are one of most dramatically modified elements of the world's natural environment. Many of the world's rivers are affected by dams and water regulation, leading to widespread environmental degradation. More than 75,000 dams fragment the rivers of the United States of America, affecting 98% of our 3.5 million miles of streams. Research evaluating how dams impact river systems has typically focused on the major impacts associated with large dams, yet large dams represent less than 1% of the structures on American rivers.

The objective of this study was to assess how the channel system of the Elwha River of Washington State responded to the construction of two hydroelectric dams of moderate size in the early 1900s. The methodology involved evaluating historical aerial photography of the Elwha River to examine spatial change over time, complemented by a detailed field study of current river system dynamics. While several geomorphic variables responded at different rates to dam installation, the general pattern following dam closure was a simplification of the channel system and the associated riparian community. Finally, the implications of the scientific findings for potential restoration of the Elwha River by dam removal will be discussed. With impending dam removal to 'restore' the damaged ecosystem and fisheries, it is particularly important to evaluate how the dams have changed the Elwha River system and how the river is currently functioning with the structures in place.