USGS Washington Water Science Center
|U.S. Department of the Interior
U.S. Geological Survey
Release: September 7, 2000
253-428-3600, ext. 2611
What will happen to 70 years of sediment built up behind the Elwha River dams in northwestern Washington State when the dams are removed?
Secretary of the Interior Bruce Babbitt and Congress have now agreed to remove the Elwha and Glines Canyon Dams and help restore the great salmon runs of the past. In preparation, a 1994 drawdown experiment was conducted to see how the sediment and the river would react to the change. The results of the experiment have just been published in a new report from the U.S. Geological Survey, Department of the Interior.
"Before the dams actually do come down, our sediment data should give planners a good idea of what to expect," said Dave Kresch, a USGS co-author of the report. After Congress enacted a 1992 law to restore the river's ecosystem and its salmon and steelhead runs, Secretary Babbitt determined that the best way to restore the river--once the home of 100-pound Chinook salmon--was to remove the dams. But no one could predict with confidence how the accumulated sediment, which had formed deltas at the upper ends of the lakes behind the dams, would react to the change in the river. So investigators from the USGS, the Bureau of Reclamation, and the National Park Service teamed up to embark on an experiment to draw down Lake Mills behind Glines Canyon Dam and record the results.
Lake Mills was gradually lowered 18 feet over the period of a week to expose the delta to the Elwha River's erosive forces. As the delta became exposed, the river cut a channel into it, leaving high, unstable banks. During the following week, the river rapidly eroded the unstable channel banks by meandering across the delta until most of the delta had disappeared. Maximum lateral erosion rates of about 80 feet per day were observed. Most of the eroded sediment was redeposited farther downstream into a new delta lobe.
To support plans to manage the sediment after the dams are removed, sediment concentrations and sediment transport rates were measured. The maximum concentration of suspended sediment measured in the Elwha River at the delta was 6,110 milligrams per liter, and the flow in the river at this time was about 600 cubic feet per second--a sediment transport rate of nearly 10,000 tons of sediment per day. While the lake was lowered, approximately 300,000 cubic yards of delta sediment deposits were eroded and redeposited further downstream. Studies conducted by other Federal agencies have projected the potential downstream fate of transported sediment. Potential impacts include increases of fine sediment concentrations during high flows, riverbed buildups of coarse sediment in the upper and lower river reaches, and enlargement of the river delta to predam conditions.
Last year another USGS study that dealt with the issue of removing the dams concluded that restoring salmon runs to the Elwha River system will greatly benefit the ecosystem (M.D. Munn, R.W. Black, and others, 1999). New runs of spawning salmon will contribute huge new quantities of nutrients from their decaying bodies. Thanks to the restored instream habitat, the nutrients will be retained long enough to nourish the next generations of salmon so that their numbers and sizes will increase.
This earlier USGS report, An Assessment of Stream Habitat and Nutrients in the Elwha River Basin: Implications for Restoration, by M.D. Munn, R.W. Black, A.L. Haggland, M.A. Hummling, and R.L. Huffman, is published as U.S. Geological Survey Water-Resources Investigations Report 98-4223. It is available at the same outlets as the new report.
The new report, Hydrologic Data Collected During the 1994 Lake Mills Drawdown Experiment, Elwha River, Washington, by Dallas Childers, D.L. Kresch, S.A. Gustafson, T.J. Randle, J.T. Melena and Brian Cluer, is published as U.S. Geological Survey Water-Resources Investigations Report 99-4215. It is available for reading at the U.S. Geological Survey office at 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402. It can be purchased from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225-0286, telephone (303) 202-4610.
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