PS 34-244 - Lessons learned from flood impacts to habitat improvement efforts after dam removal: Process-based vs form-based restoration efficacy.

Abstract: Dam removal in the western United States is rapidly occurring and restoration design methodologies being employed for post dam removal are predominantly conventional form-based techniques. These are resource intensive activities that import materials, use heavy equipment, and require engineering, which all add up in dollars spent and a bigger carbon footprint. There is very little research on using process-based restoration in conjunction with form-based designs on post dam removal projects.

Dry Creek is a 114.6 mi² watershed draining southwesterly into the Bear River in the Northern Sierra Nevada foothills. Dam removal on Dry Creek started in early July of 2020 with form-based restoration techniques used to provide anadromous fish passage and improve habitat. A single artificial low flow channel was constructed to provide fish passage under seasonal low flow periods. On October 24, 2021, a rain event measured at 4,400cfs— roughly 3,500cfs above the average peak October flows—blew out the restoration efforts.

Our goal for this project is to gauge treatment success and examine first-year pre- and post-implementation vegetative, wildlife, salmonid, hydrogeomorphic, water quality, and benthic macroinvertebrate data to quantify system recovery. We present several parameters of water quality, benthic macroinvertebrates, and physical habitat in control versus treated areas and above and below natural beaver dams in control and treatment areas. We then discuss lessons learned from historic flood impacts on artificial floodplain and channel construction.

Preliminary data show that dissolved oxygen significantly declined at all sites in the summer, as is typical with seasonal trends. Reach 2 (old lakebed) recovered more quickly and remained slightly higher. This is likely due to the habitat being primarily aerated riffle and glide, while the stagnant water in the beaver ponds may exhibit declines due to algae and other general biotic consumption of the oxygen. Temperatures exceeded salmonid recommendations in the summer months but were within or below recommended temperature ranges during migration and spawning seasons. Preliminary data suggest that benthic macroinvertebrates % EPT (Ephemeroptera, Plecoptera, Trichoptera, which are useful indicator taxa of “good” water quality and substrate) was higher in riffles in the Spring (“peak flow”) and lower in riffles in the fall (“base flow”). Reach 1 had the highest EPT during base flow, while Reach 2 had the highest EPT during peak flow. Results demonstrate rapid and notable impacts of beaver dams and their implications for understanding potential restoration success.