River-Lab

Our post-project monitoring research focuses on a natural beaver dam on French Creek in the Scott River Watershed in Siskiyou County, CA. The dam exists on a stretch of the creek with several beaver dam analogues (BDAs), which have been implemented by the Scott River Watershed Council in the hopes of combatting incision in the system and recruiting beaver to maintain the structures. French Creek in particular is an important spawning ground for the threatened coho salmon, so the impacts of the dam on fine sediment are of particular interest. Since the installment of the BDAs on French Creek, a beaver family has colonized the site and built a natural beaver dam. In 2021, high flows led to a breach in this dam. Scott River Watershed Council (Council) developed plans to fortify it using similar techniques used in the nearby BDAs. These plans will be implemented in November 2023, one week after this survey.

This dam breach presented the unique opportunity to analyze the surrounding area before its reconstruction, providing both post-breach and pre-reconstruction information on beaver presence, groundwater levels, channel morphology, and sediment grain size. We found evidence of continued beaver activity at the site. Groundwater levels were lower compared to pre-breach data from 2020. Sediment grain size was bimodal directly below the breached dam as compared to an upstream site with imported gravels, potentially indicating a release of fine sediment during the breach. Given the impact of fine sediment on salmon spawning, these fluxes of sediment should be carefully studied and managed. Comparable cross-section data did not exist for the studied reach, so we cannot draw conclusions about the impact of the breach on these metrics, though they may be used going forward to assess the influence of the artificial dam repair.