Seismic Safety of Scott Dam on Eel River
"Seismically risky dam conditions contribute to uncertainty for future water supply"
In March 2023, PG&E notified local agencies that
"out of an abundance of caution and in the interest of community safety, PG&E has made the decision to keep the spillway gates atop Scott Dam at Lake Pillsbury in Lake County open this spring and in future years. This will result in lower summer lake levels and less water being released from Lake Pillsbury later in the year. Water availability moving forward is expected to be similar to dry year conditions experienced in 2020 and 2021.
"The safety of our hometowns is PG&E’s most important responsibility, and this includes the company’s dedicated team of geoscience and engineering experts regularly performing updated seismic analyses and modeling of its dams based on regional and site-specific data. While Scott Dam is not at risk of imminent failure, models with updated parameters show an increased risk around seismic performance. As a result, PG&E is taking steps to reduce the risk.
"We recognize the impacts that reduced water storage in Lake Pillsbury will have. We’re committed to public and environmental safety and are committed to working closely with resource agencies and water districts to address any impacts associated with this safety measure.
"Again, the seismic risks for the Scott Dam remain low and the decision to not close the spillway gates helps reduce risk."
PG&E usually closed the Scott Dam gates in April of each year, allowing spring runoff and snowmelt to raise water levels in Lake Pillsbury to be released throughout the summer and early autumn for Eel River flows. A small portion of the flows were historically diverted through the Potter Valley Project to generate power, with the water being released in the East Fork Russian River in Potter Valley. This water made its way downstream, into and through Lake Mendocino and into the mainstem Russian River. Over the years, the State Water Resources Control Board issue water rights for this diverted water to water users in Mendocino and Sonoma Counties.
On April 27, 2023, PG&E held a Town Hall Meeting on zoom, providing more information on the announcement and of the plan to decommission the Potter Valley Project. It was noted the Division of Safety of Dams (DSOS) under the California Natural Resources Agency Department of Water Resources had concurred with the PG&E's plan for reducing water storage levels in Lake Pillsbury (behind Scott Dam.) PG&E stated it planned to proceed with DSOS processes according to seismic studies and risk analysis.
PG&E's announcement based on updated seismic analysis of Scott Dam suggested a higher level of risk than previous evaluations, prompting a change in operations. PG&E would now release more water from Lake Pillsbury throughout the rainy season, allowing it to flow into the Eel River this spring instead of keeping it behind Scott Dam. This resulted in less water being available for summer and autumn releases, including diversions to the Russian River watershed.
The following documents provide further background and information:
2023, 3-21 Thompson let to FERC re Seismic Scott Dam impacts.pdf2023, 4-28 FERC to PGE, Seismic Stability and IRRM Letter.pdf2023, 6-13 PGE Seismic report comment response letter.pdf2023, 3-17 PGE let to FERC on Seismic analysis of Scott Dam.pdf2023, 3-28 FERC response to PGE Seismic Scott Dam actions.pdf2023, 6-30 FOER Response to 4-3-23 Lake County Comments on Scott Dam ops.pdf2023, 4-3 Lake County letter to FERC re Seismic issue.pdf2024, 4-23 Request for Review of CEII Filings regarding PVP.pdf2023, 6-13 PGE Seismic report comment response letter attachment.pdf2023, 4-12 DSOD to PGE, Scott Dam seismic.pdf2023, 6-12 FERC reminder of report due.pdf2024, 7-1 PGE response to FERC RE-Scott Dam safety.pdf2023, 3-17 Press Dem - Scott Dam gates to stay open.pdf2023, 10-18 PG&E Scott Dam DSOD inspection report.pdf2023, 10-18 PG&E Van Arsdale Dam DSOD inspection report.pdf