Quantifying Channel Morphology Changes in Response to the Removal of the Glines Canyon Dam, Elwha River, Washington

Abstract:

The removal of two dams on the Elwha River, Washington, is the largest dam-removal project in history. Our research documents the sediment deposition, erosion, and channel changes between the dams following the initial sediment release from the removal of the upstream Glines Canyon Dam. Within the first year following the dam removal, the pulse of coarse sediment and large woody debris propagated downstream well over 6 km below the dam. The sediment deposition and altered channel hydraulics caused lateral channel migration where anabranching channels merge around new mid-channel bars and at large bends in the river channel. Documenting the river channel response to this exceptional sediment pulse could improve models of the impacts of future dam removals on similar gravel-bed rivers.

We quantified the sediment flux and channel changes at four field sites 2-6 km downstream of Glines Canyon Dam. Topographic changes were surveyed with a terrestrial laser scanner (TLS) on an annual basis from August 2012 – August 2014 and the surface sediment distribution was quantified with bimonthly sediment counts. Differencing the annual TLS data yielded an overall increase in sediment throughout the study reach, with a minimum of 20,000 m³ of deposition on bars and banks exposed above the water surface in each 700-m-long TLS survey reach. The surface sediment distribution decreased from ~18 cm to < 1 mm. Large woody debris transported downstream from the former reservoir contributed to the formation of new sand and gravel bars along the channel margin at two sites as well as the longitudinal growth of several bars throughout the study area. The new bar formations have continued to propagate downstream as new sediment and woody debris have been added and remobilized, increasing the complexity of the river channel. By spring 2013, channel features that were present before the dam removal began to re-emerge due to the remobilizing of sediment through the system.