Geomorphic evolution of a gravel‐bed river under sediment‐starved vs. sediment‐rich conditions: River response to the world's largest dam removal

Journal of Geophysical Research F: Earth Surface
By: , and 

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Abstract

Understanding river response to sediment pulses is a fundamental problem in geomorphic process studies, with myriad implications for river management. However, because large sediment pulses are rare and usually unanticipated, they are seldom studied at field scale. We examine fluvial response to a massive (~20 Mt) sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States, in an 11‐year before‐after/control‐impact study of channel morphology and grain size. We test the hypothesis that for a given flow magnitude, greater geomorphic change occurs under sediment‐rich conditions than under sediment‐starved conditions. Channel response to flow forcing was significantly different during the sediment‐pulse peak, 1–2 years after dam removal began, than earlier or later. During peak sediment supply our hypothesis was supported; major geomorphic change occurred under low flows and unit stream power ≤60 W/m2. However, by 4–6 years after dam removal began, rates of geomorphic change and sensitivity to stream power had decreased substantially such that our hypothesis was no longer unequivocally supported. These findings are consistent with a two‐phase conceptual model of dam‐removal response, involving a transport‐limited state followed by a more supply‐limited state. From comparisons with other dam removals and natural sediment pulses, we infer that the longevity of sediment‐pulse signals in gravel‐bed rivers depends upon gradient, river discharge, valley morphology, and sediment grain size. Stream power associated with substantial geomorphic change varies with sediment supply, such that assigning a general threshold stream power to gravel‐bed rivers may be untenable.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Geomorphic evolution of a gravel‐bed river under sediment‐starved vs. sediment‐rich conditions: River response to the world's largest dam removal
Series title Journal of Geophysical Research F: Earth Surface
DOI 10.1029/2018JF004703
Volume 123
Issue 12
Year Published 2018
Language English
Publisher AGU
Contributing office(s) Pacific Coastal and Marine Science Center
Description 32 p.
First page 3338
Last page 3369
Country United States
State Washington
Other Geospatial Elwha River