CHANNEL EVOLUTION ON THE LOWER ELWHA RIVER FLOODPLAIN, WASHINGTON, 1939 TO 2007

The Elwha River, Washington, USA, is being studied intensively in anticipation of a major dam-removal and river-restoration project in its watershed. The 833-km2 drainage basin is located predominantly in steep, mountainous terrain, largely within Olympic National Park. The Elwha and Glines Canyon Dams were completed on the Elwha River in 1914 and 1927 and reach 32 m and 64 m high, respectively; geomorphic and ecological alterations, including decline in native anadromous fish populations, have been attributed to effects of the dams. Downstream of both dams, the morphology of the lowermost 7 km of the river (an anabranching channel and vegetated alluvial floodplain) is expected to change substantially in response to new influx of reservoir sediment following dam removal (~4.6–6.3 x 10⁶ m³). Monitoring efforts on the lower river and floodplain before dam removal focus in part on evaluating rates and patterns of channel evolution, for eventual comparison with post-dam-removal geomorphic adjustment. Aerial photographs indicate that the channel on the lower floodplain has migrated on the order of tens to hundreds of meters, locally, in the dammed system between 1939 and 2006, with annualized migration rates as high as ~7 m/yr). Biannual field surveys that began in 2006 in three study reaches on the lower river, and in a control reach upstream of both dams, allow resolution of topographic and grain-size evolution in the channel in response to seasonal flow variations. Between September 2006 and April 2007, the lower 0.5 km of the channel migrated eastward approximately 10 m and the bed aggraded by as much as 1 m (sand and gravel deposits). Resolving seasonal rates of channel change in the dammed Elwha River will provide valuable data used to understand fluvial response to a substantial increase in sediment load following dam removal.