QUANTIFYING SEDIMENT DISTRIBUTION AND LARGE WOODY DEBRIS FOLLOWING THE REMOVAL OF THE GLINES CANYON DAM ON THE ELWHA RIVER, WASHINGTON

The removal of the Glines Canyon and Elwha Dams on the Elwha River, Washington, is the largest dam-removal project in history. Our research documents the fluctuation of large woody debris and sediment distribution between the dams following the initial sediment release from the removal of the upstream Glines Canyon Dam. The surface sediment distribution and the amount, length and position of large woody debris were documented over a period of 1 year at four survey sites. Previous observations on sediment reservoirs that are one-tenth the size of the Glines Canyon reservoir have shown that the majority of the sediment deposition following a dam removal occurs within the first 2 km below the breach. In contrast, we anticipated that within the first year following the removal of the upstream dam on the Elwha River the initial pulse of sediment and large woody debris would propagate several kilometers downstream.

We employed various methods to quantify the flux of large woody debris and the sediment distribution within the study area. We measured sediment greater than 1mm along with photographic documentation on a bi-monthly basis at 15 survey transects in the middle reach from October 2012-August 2013, before and after the initial coarse-sediment release. The large woody debris was quantified and mapped with ArcGIS 10.2 on four aerial orthographic photographs from the National Park Service taken on different dates between August, 2012 and August, 2013. The areas of logjams consisting of four or more logs in close proximity to one another were mapped, as was the location and length of individual logs exceeding 2 m.

The quantity of large woody debris and logjams increased from August 2012-June 2013, during the high-flow season, and decreased thereafter in the low-flow season. In contrast, some logjam areas continued to increase throughout the entire period, indicating a possible consolidation of the debris. The pulse of sediment released from the reservoir abruptly decreased the mean sediment size from approximately 18 cm to < 1 mm by rapid burial of the bars along the channel margin. Large woody debris transported downstream has caused new sand and gravel bars to form along the channel margin at two of our study sites, as well as the longitudinal growth of 5 – 10 bars throughout the study area.