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Sedimentology of new fluvial deposits on the Elwha River, Washington, USA, formed during large-scale dam removal

River Research and Applications

By:
and
DOI: 10.1002/rra.2724

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Abstract

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river-restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi-stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal-formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre-dam-removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel-margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre-dam-removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer-term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river-restoration efforts where large dam removal is planned or proposed.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Sedimentology of new fluvial deposits on the Elwha River, Washington, USA, formed during large-scale dam removal
Series title:
River Research and Applications
DOI:
10.1002/rra.2724
Volume
31
Issue:
1
Year Published:
2015
Language:
English
Publisher:
John Wiley & Sons
Publisher location:
Chichester, UK
Description:
20 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
River Research and Applications
First page:
42
Last page:
61
Number of Pages:
20
Country:
United States
State:
Washington
Other Geospatial:
Elwha River
Online Only (Y/N):
N
Additional Online Files(Y/N):
N