The dams of the Elwha River, Washington: Downstream impacts and policy implications

Dams and river regulation have dramatically modified the world's rivers, leading to widespread environmental degradation. In the United States, less than 2% of streams remain in natural conditions. While river managers and environmental policy are increasingly emphasizing the mitigation of these impacts, restoration of rivers requires a sound understanding of how dams have altered the physical, chemical, and biological integrity of streams. This study examines the downstream influence of two dams on the hydrology and channel system of the Elwha River of Washington state. Hydrologic modification was measured using U.S. Geological Survey stream gage records. These data indicate that Glines Canyon dam had a substantial impact on streamflows, especially low flows, for the first two decades following dam closure. However, for the past half a century, the dam has been operating with a run-of-the-river goal and hydrologic alteration has been visible but not of great magnitude. The downstream channel system of the Elwha River has responded to the paucity of sediment and modified hydrology associated with dam installation and operations. Analysis of aerial photography since 1938 demonstrated that the Elwha River adjusted its sinuosity and active channel area in response to natural controls and human activities. Most historical human impacts constrained the river spatially; either indirectly through controlled flow releases through the dams or directly through channelization and levee and dike construction. An evaluation of the current channel system, based on cross section surveys and physical field data, indicates that the dams have created three segments of the lower Elwha River which are functioning differently and have distinct channel morphologies and riparian community structures. The channel was the most unstable upstream from the dams, very stable between the structures, and less stable downstream from Elwha Dam. Channel stability was found to be associated with a reduction in geomorphic and biotic complexity. While current policy goals are to "fully restore" the Elwha River ecosystem by removing the dams, this study indicates that full restoration is not scientifically feasible given the past and current impacts on the river system. The restoration goal should be modified to aim for a more natural condition, with specific objectives that acknowledge that recovery will be spatially variable.