Text / Channel evolution on the dammed Elwha River, Washington, USA

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Title: Channel evolution on the dammed Elwha River, Washington, USA
Alternative Title: Geomorphology 127 (2011) 71-87
Creator: Amy E. Draut, Joshua B. Logan, Mark C. Mastin
Date Created and/or Issued: 9/13/10
Publication Information: Elsevier B.V.
Contributing Institution: UC Riverside, Library, Water Resources Collections and Archives
Collection: Clearinghouse for Dam Removal Information (CDRI)
Rights Information: Copyrighted
Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user.
Description: Scope/Content: Abstract: Like many rivers in the western U.S., the Elwha River, Washington, has changed substantially over the past century in response to natural and human forcing. The lower river is affected by two upstream dams that are slated for removal as part of a major river restoration effort. In preparation for studying the effects of dam removal, we present a comprehensive field and aerial photographic analysis of dam influence on an anabranching, gravel-bed river. Over the past century with the dams in place, loss of the upstream sediment supply has caused spatial variations in the sedimentary and geomorphic character of the lower Elwha River channel. Bed sediment is armored and better sorted than on the naturally evolving bed upstream of the dams. On time scales of flood seasons, the channel immediately below the lower dam is fairly stable, but progresses toward greater mobility downstream such that the lowermost portion of the river responded to a recent 40-year flood with bank erosion and bed-elevation changes on a scale approaching that of the natural channel above the dams. In general, channel mobility in the lowest 4 km of the Elwha River has not decreased substantially with time. Enough fine sediment remains in the floodplain that - given sufficient flood forcing - the channel position, sinuosity, and braiding index change substantially. The processes by which this river accesses new fine sediment below the dams (rapid migration into noncohesive banks and avulsion of new channels) allow it to compensate for loss of upstream sediment supply more readily than would a dammed river with cohesive banks or a more limited supply of alluvium. The planned dam removal will provide a valuable opportunity to evaluate channel response to the future restoration of natural upstream sediment supply.
Scope/Content: Dam type: concrete
Scope/Content: Height: 105 & 210 feet.
Scope/Content: Date constructed: 1910. Date removed: 2011.
Type: text
Identifier: ark:/86086/n22j6bfb
1176
Subject: Sediment and channel dynamics
Dams
Dam retirement
Place: Elwha, WA
Elwha and Glines Canyon dams

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