Quantifying sediment distribution and channel changes following removal of the Glines Canyon Dam, Elwha River, Washington

The ongoing removal of two dams on the Elwha River, Washington, beginning in 2011, is the largest dam-removal project in history. Our research quantifies sediment deposition and channel changes in the reach between the dams during the removal of the upstream Glines Canyon Dam. After the initial release in October 2012, the sediment transport and deposition throughout our study reach has had a dynamic effect on channel and bar morphology. Multiple methods were used to quantify the changes in the morphology of the channel and gravel bars. Detailed topographic surveys using Terrestrial Laser Scanning (TLS), and total station bathymetric surveys began before the initial sediment release and were repeated one year later to quantify changes in sediment volume and distribution. Monthly surface pebble counts and photo documentation were used to quantify changes in surface grain-size distribution and the effects of the influx of sediment and woody debris on the channel morphology. We measured changes in channel morphology and grain-size distribution at four field sites located 2 - 6 km downstream of Glines Canyon Dam. In the first two months following the initial sediment release, the mean sediment size on the surface of the channel bars abruptly decreased from approximately 18 cm to < 1mm due to rapid burial by new sediment. Large woody debris transported downstream from the former reservoir 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. The copious amount of sediment transported through the system since October 2012 also created new mid-channel bars at all four field sites. In some locations, the formation, growth and merging of bars has effectively narrowed the river channel, causing it to migrate laterally and erode the bank. The initial sediment deposition has started eroding at the downstream end of some of the new bars since May 2013. This localized erosion is related to the collection of new woody debris that is changing the flow pattern of the channel. Concurrent with sediment bar growth and formation, sediment has also filled mainstem pools and transformed the pool-riffle profile to a continuous high-energy glide. As the sediment and woody debris wave has progressed downstream, the channel stage has increased in response. Although there were no large floods, the high winter flows and spring runoff have continued to transport sediment throughout the middle reach. By spring 2013, portions of the pre-sediment features began to re-emerge due to reduction of new sediment flowing through the system. We anticipate that the bar formations in this reach will continue to evolve as the dam removal is completed in the next year and high seasonal discharges introduce and remobilize the sediment.