Project Specifics

The California State Coastal Conservancy (SCC) and others are exploring alternatives, including dam removal, for relicensing the Potter Valley Hydropower Project in northern California.

The Upper Eel River contains two dams, their reservoirs, and a diversion tunnel that sends water to the Russian River watershed, currently owned and operated by Pacific Gas and Electric Company (PG&E). Because the Eel River has the highest average suspended sediment load per unit area of any river of its size or larger in the contiguous U.S., the SCC retained Geosyntec to design and execute a Sampling and Analysis Program to characterize the sediments that have deposited in Lake Pillsbury and the Van Arsdale Reservoir, the reservoirs behind the two dams. The goal was to evaluate the potential impact of the sediments trapped behind the dams on downstream water quality upon dam removal.

Geosyntec Scope of Services

Geosyntec prepared a Sampling and Analysis Plan (SAP), conducted a 10-day field program, and executed a sampling event at the remote site located along the upper Eel River. The team collected bathymetry, water quality, and sediment data and, based on these site factors, exploration of dam removal as an alternative to relicensing of the facility is being conducted. In addition, our team collected physical data from Lake Pillsbury and Van Arsdale Reservoir as part of this program to support the future development of a hydrodynamic and sediment transport model to estimate sediment transport and redistribution following dam removal.

The second phase of Geosyntec’s scope is to support a geotechnical evaluation and feasibility analysis that focuses on model predictions of how the sediment will be evacuated from the reservoir under different notching/demolition options. Our team is developing a predictive model to estimate downstream suspended sediment concentrations over time for the various dam notching/demolition options. We will then compare this evaluation of suspended sediment concentrations to literature values of stressful and lethal thresholds for fish and other aquatic species to assess the potential impacts of different dam notching/demolition scenarios.

Notable Accomplishments

As further studies and negotiations continue over whether and when to remove the two dams continue, Geosyntec’s detailed analysis of the potential impacts of downstream sediment transport in the Eel River based on different dam removal strategies will serve as a critical factor in protecting fish and other aquatic and related populations. SCC, with whom we are working hand in hand throughout the process, including conducting sampling and other field work, is thoroughly pleased with our work, commenting in one case, “Fantastic day with an awesome crew!”

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