Introduction

The two dams on the Elwha River in Clallam County, Washington (fig. 1) are scheduled for removal in the near future: the Elwha Dam at river kilometer (RK) 7.9, which forms Lake Aldwell, and the Glines Canyon Dam at RK 21.6, which forms Lake Mills. Both dams were constructed early in the 20th century without provision for fish passage and largely are responsible for the loss of anadromous fish runs, which historically occurred throughout the year. In 1992, restoration of the river was mandated by Congress under the Elwha River Ecosystem and Fisheries Restoration Act (Public Law 102-495). By 1995, the Secretary of the Interior decided that removal of both dams was a necessary step toward river restoration (U.S. Department of the Interior, 1995). Because most of the Elwha River basin is within the Olympic National Park, the National Park Service, the primary Federal agency responsible for the preservation and protection of this land, was tasked with removing the dams to allow the full restoration of native anadromous fisheries and the broader Elwha River ecosystem.

Since their construction, both dams have accumulated sediment that otherwise would have been transported downstream. The volume of retained sediment has been estimated at about 10.6 million cubic meters in Lake Mills and 3.1 million cubic meters in Lake Aldwell (Childers and others, 1999). The removal of both dams on the Elwha River will be one of the largest dam removal projects ever attempted and the release of stored sediment by the free-flowing river will result in profound changes to the channel morphology and habitat of the lower river. In cooperation with the National Park Service, the U.S. Geological Survey (USGS) made pre-dam-removal measurements of suspended-sediment load during water years 2006 and 2007. This information will provide a baseline of suspended-sediment data useful to planners and scientists monitoring geomorphic and habitat changes in the river as it reaches a dynamic equilibrium after the dams are removed.

Description of Study Area

The Elwha River is about 70 km long and its headwaters are in the central part of the Olympic Mountains at an altitude of about 1,350 m. The river flows from the mountains generally northward, passes through the two dams, crosses a coastal terrace and discharges into the Strait of Juan de Fuca (fig. 1), the waterway that connects Puget Sound with the Pacific Ocean.

As is characteristic of rivers on the Olympic Peninsula, the Elwha River is fed seasonally by varying contributions from snowmelt, groundwater discharge, and precipitation runoff. Eighty-three percent of the Elwha River basin is within the Olympic National Park and includes 830 km² of mostly forested land with many areas of pristine wilderness. Most of the basin is within the eastern core of the Olympic Mountains where accreted marine sediments in the form of sandstone and shale formations predominate (Tabor and Cady, 1978). The climate in the Elwha River basin is maritime with relatively wet, mild winters and dry, cool summers. Annual precipitation in the basin ranges from about 560 cm at the upper elevations to about 140 cm near the mouth of the river (Munn and others, 1998).

Purpose and Scope

This report provides suspended-sediment concentrations measured during water years 2006 and 2007 (October 1, 2005–September 30, 2007) at two USGS gaging stations on the Elwha River: Elwha River above Lake Mills (station no. 12044900) upstream of Elwha and Glines Canyon dams, and Elwha River at McDonald Bridge (station no. 12045500) between the dams. Data for suspended-sediment and bedload measurements made by the USGS during water years 1995–98 at the gaging station upstream of Lake Mills also are included. Using all available data at this station, regression models were developed for estimating suspended-sediment discharge and bedload discharge as a function of streamflow. These models were used to estimate monthly and annual sediment loads. At the McDonald Bridge gaging station, daily suspended-sediment concentrations from pumped samples were combined with the record of streamflow to calculate daily, monthly, and annual suspended-sediment loads. The difference in loads upstream and downstream of Lake Mills was used to compute the trap efficiency of the reservoir. Collectively, these measurements of suspended-sediment provide a baseline for monitoring change in sediment transport following dam removal.