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Final Beach-Erosion Survey of the Elwha River Delta Before Dam Removal
On August 26-27, 2011, the U.S. Geological Survey (USGS) conducted its final beach-erosion survey of the Elwha River delta before a historic dam removal began upstream in September. The survey is part of an ongoing study of how damming has affected the ecosystem.
Two dams on the Elwha River—the Elwha and the Glines Canyon—have stopped most of the flow of sediment to the beaches on its delta for nearly 100 years. Historical photographs and topographic-survey data document severe erosion on these beaches, corroborated by 7 years of Global Positioning System (GPS) beach surveys conducted on foot and with personal watercraft by the USGS in collaboration with the Lower Elwha Klallam Tribe and the Washington State Department of Ecology.
Not only have the beaches eroded quickly during the past 100 years, but erosion rates have increased significantly over time. The greatest changes have been observed along the tribal reservation, where USGS scientists and their collaborators reported in a 2009 article that erosion averaged nearly 2 ft per year between 1939 and 2006 (see Warrick and others, Geomorphology, v. 111, no. 3-4, p. 136-148, http://dx.doi.org/10.1016/j.geomorph.2009.04.012). Most of the erosion has occurred along the primary pathway by which river sediment moves into and along the beach. The combination of the rapid beach erosion and the reduction of river sediment by the dams lends evidence that the dams are responsible for the beach erosion.
With erosion, other beach characteristics have also changed, and part of the USGS work is to study the changes' effects on wildlife habitat. For example, the lowest area of the beach east of the river mouth is mostly cobble, which is inconsistent with tribal oral histories suggesting that abundant shellfish were harvested from this beach. Erosion of the beach has apparently left behind only the largest and heaviest rocks.
The removal of the two dams on the Elwha River, which began September 17, 2011, will take about 2½ years to complete. As dam removal proceeds, new supplies of sediment will flow downriver to the beaches of the river delta. Although only part of the approximately 20 million m3 of sediment now trapped behind the dams will move downstream, this sediment will likely slow—or even reverse—the recent trends of erosion on the beach.
The USGS plans to continue these surveys for several years after the dam removal to evaluate how a beach responds to restoration of its sediment supply. Such studies are important to evaluate the effectiveness of this and future coastal-restoration efforts.
The Elwha River Restoration Project, created by act of Congress in 1992, aims at the full restoration of the Elwha River ecosystem and the native fish that ascend the river from the sea to breed. (To learn more, visit http://usgs.gov/elwha/ and http://www.nps.gov/olym/naturescience/elwha-ecosystem-restoration.htm; see also Elwha River Dam Removal—Rebirth of a River, USGS Fact Sheet 2011-3097, http://pubs.usgs.gov/fs/2011/3097/.)
in this issue:
Final Beach-Erosion Survey Before Elwha River Dam Removal
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