River channel response to the removal of the Pilchuck River Diversion Dam, Washington State

Northwest Science
By: , and 

Links

Abstract

In August 2020, the 3-m tall Pilchuck River Diversion Dam was removed from the Pilchuck River, allowing free fish passage to the upper third of the watershed for the first time in over a century. The narrow, 300-m long impoundment behind the dam was estimated to hold 4,000–7,500 m3 of sand and gravel, representing less than one year's typical bedload flux. Repeat cross section surveys, stage sensors, and time-lapse cameras were used to document the physical channel response over the first year following dam removal. A total of 7,400 m3 (effectively 100%) of impoundment sediment was eroded in the first year, with 25% accomplished by manual regrading during dam removal. Most river-caused erosion occurred during a sequence of modest flows in October 2020. Downstream deposition totaled 4,300 m3, predominately filling in the first 100 m downstream of the dam site. Deposition tapered below detectable levels within 350 m, and most downstream channel adjustments occurred before November 2020. Multiple high flows after December 2020 caused little upstream or downstream change. The physical river response to this dam removal then appears to have been largely accomplished within several months by modest flows, consistent with pre-removal modeling and observations from other regional dam removals. Efficient sediment evacuation was likely aided by the narrow and steep-walled impoundment geometry. Our observations support existing guidance that the physical river response to small dam removals is typically rapid and modest; the benefits of removal may then often be gained with minimal negative downstream geomorphic impacts.

Study Area

Publication type Article
Publication Subtype Journal Article
Title River channel response to the removal of the Pilchuck River Diversion Dam, Washington State
Series title Northwest Science
DOI 10.3955/046.097.0113
Volume 97
Issue 1-2
Year Published 2024
Language English
Publisher Northwest Scientific Association
Contributing office(s) Washington Water Science Center
Description 12 p.
First page 134
Last page 145
Country United States
State Washington
Other Geospatial Pilchuck River
Google Analytic Metrics Metrics page
Additional publication details