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

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 reach 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. A series of 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. 7,400 m3 (~100%) of impoundment sediment was eroded in the first year, with 25 percent 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.