Water Levels at Lake Pillsbury to Be Lower this Year
Despite 2023 starting off as a normal or above-normal water year, the spillway gates atop Scott Dam at Lake Pillsbury in Lake County will not be closed this spring or in future years. In years past, PG&E has closed the gates at the dam on or around the beginning of April, allowing lake levels to increase by as much as 10 feet above the spillway crest during the spring runoff, storing up to about 20,000-acre feet of additional water to support recreation and releases during the dry season.
For the Eel River, this means more water will potentially flow in the spring as PG&E is not storing as much water with the gates open. It also means less water will be stored in Lake Pillsbury for summer and fall water releases that support cold water fishery resources in the Eel River below Scott Dam and Russian River agricultural interests, as well as for recreation, specifically boat ramp access.
PG&E has made the decision out of an abundance of caution and in the interest of community safety. Keeping the spillway gates open at Scott Dam from this point forward allows us to reduce potential risk around seismic performance. Seismic performance refers to the structure’s ability to meet safety and serviceability requirements during and after an earthquake.
As part of PG&E’s commitment to safety, the company’s team of geoscience and engineering experts regularly performs updated seismic analyses and modeling of its dams based on regional and site-specific data. PG&E also retains independent consultants to review its seismic analyses for consistency with current industry standards and practices. New information and updated analyses suggest the level of risk around seismic performance at Scott Dam is greater than the previous evaluation. This is not an unusual circumstance as the practice of seismic engineering and geoscience understanding of earthquake potential are continually advancing and periodic due diligence assessments on potential impacts to dams are part of PG&E’s Dam Safety Program.
Even so, the probability of a seismic event causing severe damage to the dam remains relatively low on an annual basis (estimated to be on the order of a 1-in-900-year event). The most effective means of reducing risk in the near term is to store less water in the reservoir and the most feasible way to store less water is achieved by leaving the spill gates open – which in years past have been closed from April through October. Mitigating risk to seismic performance by reducing the reservoir storage is an action to meet PG&E’s safety standards and those of our dam safety regulators.
“While risks to the dam remain very low, by reducing water levels in the reservoir we can mitigate against risk,” said Jan Nimick, vice president of Power Generation at PG&E. “Storing less water in the reservoir (a 26% reduction compared to a full reservoir) lowers the water load on the dam, thereby significantly reducing the risk during or after a seismic event.”
With the dam gates remaining open, water availability will be similar to dry year conditions experienced in 2020 and 2021, when Lake Pillsbury’s spring top-off did not reach the spillway crest elevation and the project operated under FERC-approved flow variances.
The lower water levels in Lake Pillsbury will affect water customers, water availability for fishery resources protected under the ESA, and recreationists.
“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 make best use of available water, as we do in dry years,” said Nimick.
Water releases into the Eel River and East Branch of the Russian River will continue as required by the Federal Energy Regulatory Commission (FERC) license for the Potter Valley Project, or by a FERC-approved flow variance. PG&E will submit a variance request to ensure Lake Pillsbury water levels are appropriately managed this year and intends to request a license amendment to modify long-term operations pending surrender of the project. These requests will involve Section 7 consultation with resource agencies to determine Eel River flows to minimize impacts to fishery resources listed under the Endangered Species Act (ESA) and PG&E anticipates a minimum flow of 5-25 cubic feet per second to the East Branch Russian River. However, the lower lake elevation will mean less water will be available this year and in the future for water users.
PG&E is developing a FERC license surrender application for the Potter Valley Project, which will address the eventual disposition of Scott Dam and Lake Pillsbury. The Surrender regulatory process allows FERC to assess all impacts of the surrender and decommissioning. The company plans to continue to develop long-term mitigation measures which could include expedited partial or full removal of Scott Dam. PG&E will continue to study and model the seismic performance of Scott Dam to better understand and manage potential risks through the surrender and decommissioning process.
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