Summary

The two dams on the Elwha River in Washington State are scheduled for removal in the near future: the Elwha Dam, which forms Lake Aldwell, and the Glines Canyon Dam, which forms Lake Mills. In an effort to estimate suspended-sediment loads prior to dam removal, the U.S. Geological Survey (USGS) collected suspended-sediment samples during water years 2006 and 2007 at two gaging stations on the Elwha River. One gaging station is in the free-flowing river upstream of Lake Mills and the other gaging station is downstream of Glines Canyon Dam. At the gaging station upstream of Lake Mills, samples of suspended sediment were collected over a range of flows including a large peak in November 2006 when suspended-sediment concentrations exceeded 7,000 milligrams per liter, the highest ever recorded on the river. A limited size analysis of samples collected before and during this flood peak indicated that the percentage of fine sediment (less than 0.0625 millimeters) increased as the flood progressed, likely due to the magnitude and seasonal timing of the flood. Suspended sediment measurements made during this study were combined with previously published measurements made during water years 1995–98 to develop a regression equation for estimating suspended-sediment concentration as a function of streamflow. This equation was used to estimate monthly and annual suspended-sediment loads for water years 2006 and 2007 using the USGS software program Graphical Constituent Loading Analysis Software (GCLAS). Additionally, a regression equation for estimating bedload discharge was developed from measurements made during the Lake Mills drawdown study and measurements made during water years 1995–98. On the basis of these regression equations, a flow-duration approach using the streamflow record upstream of Lake Mills produced an estimate of the average total annual sediment load from the Elwha River into Lake Mills of 327,000 megagrams with a range of uncertainty of +57 to -34 percent (217,000–513,000 megagrams) at the 95 percent confidence level. Of this total, 77 percent was estimated suspended-sediment load and 23 percent was estimated bedload.

At the McDonald Bridge gaging station downstream of Glines Canyon Dam, daily suspended-sediment samples were obtained with an automated pump sampler. These samples were used in Graphical Constituent Loading Analysis Software (GCLAS) to compute a record of daily, monthly, and annual suspended-sediment loads. At this site, a time lag of about 8 hours was observed between sediment concentration and streamflow peaks due to the slower transport of sediment through the reservoir relative to the propagation of the flood peak.

A comparison between the annual suspended-sediment loads at both gaging stations determined loads of 186,000 megagrams and 233,000 megagrams in water years 2006–07 upstream of Lake Mills and 49,300 megagrams and 75,200 megagrams, respectively, for the same period downstream of Glines Canyon Dam. Substantially more sediment load measured upstream compared with downstream of the reservoir indicates the extent to which Lake Mills traps and accumulates sediment. A trap efficiency of 0.86 was determined for the Lake Mills reservoir, based on suspended-sediment loads upstream and downstream of the lake. These pre-dam-removal estimates of suspended-sediment loads and sediment-discharge relations provide a baseline of data that will be useful to planners and scientists monitoring geomorphic and habitat changes in the river as it reaches a dynamic equilibrium following the removal of the dams.