Elwha and Glines Canyon Dams, Elwha River near Port Angeles, Washington
The Elwha River flows northward from the Olympic Mountains of northwest Washington State to the Strait of Juan de Fuca near the town of Port Angeles, Washington. The upper watershed of the Elwha River is located within Olympic National Park and within a Wilderness Area (U.S. Department of the Interior, 1996). With a mean annual flow of 1,508 cubic feet per second, and a drainage area of more than 270 square miles, the Elwha River is the fourth largest river on the Olympic Peninsula. The mean annual runoff is 76 inches. The highest peak flow recorded was 41,600 cubic feet per second on November 18, 1897 and the lowest mean-daily flow recorded was 10 cubic feet per second on October 3, 1938.
Private companies constructed two large dams on the Elwha River during the early 1900’s. Elwha Dam, constructed during the period 1910-13, is a 105-foot high concrete gravity dam that forms Lake Aldwell 8 miles upstream from the river's mouth. Glines Canyon Dam, built in 1927, is a 210-foot high concrete arch dam that forms Lake Mills 13 miles upstream from the river's mouth. When the dams were first built, they were significant producers of electricity on the Olympic Peninsula. Today, the dams are operated in a run-of-the river mode and generate about 40 percent of the electricity needs for the Diashowa America paper mill in Port Angeles, Washington. These two large dams have no facilities for the upstream passage of anadromous fish and their removal would provide an opportunity to restore an entire watershed to near natural conditions (U.S. Department of the Interior, 1996).
In 2000, the U.S. Department of the Interior purchased Elwha and Glines Canyon Dams in preparation for their removal. One of the major challenges in removing these two dams is the management of nearly 18 million cubic yards of sediment trapped within the two reservoirs. Of this total sediment volume, 13 million cubic yards is trapped behind Glines Canyon Dam in Lake Mills and 5 million cubic yards is trapped behind Elwha Dam in Lake Aldwell. About one half of the sediments in Lake Mills is coarse grained (sand, gravel, and cobble-sized) and the remaining half is fine grained (clay and silt-sized). Some of the fine-grained sediments are transported through the reservoirs while the remainder is deposited along the reservoir bottoms. All of the coarse-grained sediments are trapped within the reservoirs as delta deposits located at the upstream end of each reservoir. The width of these reservoir deltas is large, about ten times greater than the width of the alluvial river channel.
The Lake Mills drawdown experiment (Childers and others, 2000) was crucial to learning about the erodibility of reservoir sediments. Lake Mills was drawn down 18 feet over a one-week period and then the lake elevation was held constant for one additional week. This drawdown experiment exposed the upper layer (12 to 18-foot thickness) of coarse delta sediment to direct erosion by the Elwha River. Several types of data were collected throughout the drawdown experiment including:
- river discharge
- suspended sediment concentration and water quality
- bed load
- particle size gradations and chemical composition of the upper delta layer
- repeat cross section measurements of the delta surface (above and below water)
- time-lapsed photography of the delta erosion
- aerial photography
During the reservoir drawdown, vertical incision through the coarse delta sediments was very rapid, but the width of erosion was much less than the reservoir delta width. The eroded sediments redeposited in the receded lake and formed a new delta immediately downstream of the initial delta front. Once the reservoir was drawn down 18 feet, the lake elevation was held constant for another week. During the period of constant lake elevation, lateral erosion processes dominated the continued erosion of the exposed delta, especially at the downstream end. The width of erosion was near zero at the delta's upstream end, but increased at a parabolic rate in the downstream direction until the erosion width equaled the reservoir width at the delta face (Randle and others, 1996).
The Lake Mills drawdown experiment provided critical data for a new sediment erosion model and helped lead to the conclusion that the reservoir sediment erosion could be managed by controlling the rate of reservoir drawdown and that not all of the sediments would be eroded from the reservoir. The new reservoir sediment erosion model was an empirically based mass balanced model (Randle and others, 1996). This model predicted the rates and final quantities of fine and coarse-grained sediments eroded from each reservoir. The standard HEC-6 model (U.S. Army Corps of Engineers, 1993) was applied, using measured cross sections of the downstream river channels, to predict the potential for sediment deposition. The selected plan for managing the 18 million cubic yards of reservoir sediment is to concurrently remove both dams, in controlled increments, over a two-year period. Rates of sediment erosion, transport, and deposition are to be intensively monitored during dam removal. These rates will be monitored to determine if they match the predicted values within acceptable limits before proceeding with the next increment of dam removal (U.S. Department of the Interior, 1996).
Childers, D.; Kresch, D.L.; Gustafson, A.S.; Randle, T.J.; Melena, J.T.; Cluer, B., September 2000, Hydrologic Data Collected During the 1994 Lake Mills Drawdown Experiment, Elwha River, Washington, Water-Resources Investigations Report 99-4215, U.S. Geological Survey, Tacoma, Washington, 115 pages.
Randle, T.J.; Young, C.A.; Melena, J.T.; Ouellette, E.M., October 1996, Sediment Analysis and Modeling of the River Erosion Alternative, Elwha Technical Series PN-95-9, U.S. Department of the Interior, Bureau of Reclamation, Boise, Idaho, 138 pages.
U.S. Army Corps of Engineers Hydrologic Engineering Center, August 1993, HEC-6 Scour and deposition in rivers and reservoirs, User's Manual, Version 4.1.
U.S. Department of the Interior, November 1996, Elwha River Ecosystem Restoration Implementation, Final Environmental Impact Statement,” Olympic National Park, Washington, 281 pages.