Channel change monitoring following the Pilchuck Dam removal

Science Center Objects

The issue:

In the summer of 2020, the Pilchuck Dam will be removed, allowing salmon access to the upper third of the Pilchuck River watershed for the first time in over a century. This removal will be associated with a short period increased sediment delivery, as the river re-works material trapped behind the dam. While the volume of impounded sediment is small and major changes in the downstream river are not expected, monitoring is needed to ensure downstream river-adjacent properties are not impacted.

How USGS will help:

In the summer of 2020, the Pilchuck Dam will be removed, allowing salmon access to the upper third of the Pilchuck River watershed for the first time in over a century. This removal will be associated with a short period increased sediment delivery, as the river re-works material trapped behind the dam. While the volume of impounded sediment is small and major changes in the downstream river are not expected, monitoring is needed to ensure downstream river-adjacent properties are not impacted.

Pilchuck Dam, July 10, 2020

Pilchuck Dam, July 10, 2020

(Credit: Scott Anderson. Public domain.)

Problem:

The Pilchuck Dam, a 3-m high diversion dam on the Pilchuck River in western Washington, is a non-functioning structure that restricts fish passage to the upper watershed. That dam is now slated for removal in the summer of 2020. The removal of the dam will generate a pulse of sediment, as the sand and gravel impounded behind the structure is eroded and mobilized downstream. Although the volume of impounded sediment is modest in comparison to natural sediment loads and downstream impacts are expected to be small, channel change monitoring in the one-kilometer reach immediately downstream of the dam is needed to identify and address any potential adverse impacts.

Objectives:

The objective of this work is to provide timely information about channel adjustments upstream and downstream of the dam over the first year following its removal.

Site of Pilchuck Dam after removal, September 3, 2020

Site of Pilchuck Dam after removal, September 3, 2020

(Credit: Scott Anderson. Public domain.)

Relevance and Benefits:

This monitoring will inform whether any adaptive management is needed and will also add to the growing body of research on river response to dam removals and, more generally, sediment pulses. This study addresses goals laid out in the Department of the Interior strategic plan for fiscal years 2018 – 2022, including goal #1 (“Utilize science in land, water, species and habitat management supporting decisions and activities”) under the “Conserving our land and water” mission area and goal #4 (“Provide science to safeguard communities from natural hazards”) under the “Protecting our people and border” mission area. This study also aligns with goal #4 (“Anticipate and respond to water-related emergencies and conflicts”) laid out for the Water Mission Area (WMA) in Everson et al., (2013) and retained through the 2018 WMA re-organization.

Approach:

Monitoring of post-removal geomorphic change on the Pilchuck River will include repeat total station surveys of cross sections and long profiles both upstream and downstream of the dam; continuous monitoring of stage as a proxy for bed elevation changes downstream of the dam; and time-lapse photography at several locations within the study area. Monitoring efforts will start in the spring prior to removal and continue through the following spring. Additional tasks include assistance in the development of an EPA Quality Assurance Project Plan (QAPP) prior to this work, and the publication of an integrated analysis of the geomorphic response to dam removal after field work is complete.