Design and Preliminary Evaluation of Beaver Dam Analogs to Reduce Downstream Sediment Loads: A Pilot Project in California Creek, Spokane, Washington, USA

Hangman Creek is one of the Spokane River's largest tributaries, draining more than 400,000 acres of agricultural and forested land across four Washington and Idaho counties. The degradation of the Hangman Creek watershed has been frequently documented, with studies showing that the creek does not meet Washington State water quality standards. As part of their beaver solution program, the Lands Council (TLC) is investigating the use of beaver dam analogs (BDAs) as a cost-effective method for improving water quality by simulating the same ecosystem benefits as active beaver dams. BDAs are channel spanning structures that are semi-porous to water, sediment, and fish, and are biodegradable temporary features that simulate the ecosystem benefits of active beaver dams, but can be implemented where habitat may not be suitable for sustaining active beaver populations. In September 2016, TLC partnered with the U.S. Fish and Wildlife Service (USFWS) and Gonzaga University to design and evaluate BDAs in California Creek, a tributary to Hangman Creek. Gonzaga University has developed a long-term monitoring plan to strategically examine the effectiveness of BDAs to reduce sediment loads in California Creek. Monitoring efforts have occurred in since 2016 and include, repeat cross section surveys, sediment characterization, soil probing, and geomorphic change detection using repeat RTK surveys and repeat structure-from-motion aerial photogrammetry analyses. Over the past two years, spring floods acted to drastically alter channel morphology and damage the BDA structures. Using the results from drone video footage during the floods and design guidelines, efforts were made to modify, repair, and design new, more-resilient, BDA structures. This paper focuses on presenting the initial results of monitoring for sediment trapping effectiveness (using repeat cross section surveys, repeat RTK surveys, and pebble count data) and the adaptive management of the BDA structures in California Creek.