MENDOCINO CO., 10/14/26 — PG&E plans to start repairing a void at the base of Cape Horn Dam by Nov. 20, according to a letter the utility filed with federal dam-safety regulators Oct. 6.
Cape Horn Dam holds back Van Arsdale Reservoir on the Eel River in Mendocino County. An intake 400 feet upstream of the dam sends Eel River water into the Potter Valley Project’s tunnels. The water runs to the Potter Valley powerhouse, which empties into the East Fork of the Russian River. PG&E is obligated to deliver up to 19,000 acre-feet of that water to the Potter Valley Irrigation District, according to the Federal Energy Regulatory Commission. The dam is also home to the Van Arsdale Fisheries Station, where salmon and steelhead climb a fish ladder past the dam.
A FERC engineer found the void during an inspection Sept. 8 and 9, 2025, according to FERC’s follow-up letter to PG&E dated Sept. 12, 2025. The letter lists two items at Cape Horn.
The first was seepage at the base of the dam’s earthen embankment that was “bypassing both weirs” PG&E uses to measure it.
The second was a void at part of the base of the dam’s downstream slope, in a rock-filled section and “under what appeared to be a layer of mortared cobbles/riprap from previous erosion repairs.” Riprap is a layer of large stone laid on a slope to guard against erosion, according to federal dam-safety guidelines. The letter said rock protecting that slope from erosion appeared to have been removed.
FERC told PG&E to “investigate the void, its extents, potential causes, and any impacts to underlying embankment materials,” to weigh any effect on dam safety and to file a plan and schedule for repairs. The same letter said the inspection found “no conditions” that “would require immediate remedial action to protect the safety of the project.”
The details of the repair aren’t public. PG&E’s work plan, its earlier plans and its June report on the seepage were all filed as critical energy infrastructure information and weren’t available from FERC’s public online library.
A November start
PG&E sent FERC its first plans on Oct. 20, 2025. On June 19, 2026, it told FERC it had dealt with the seepage item and sent a repair plan for the void. FERC accepted that plan Sept. 14, setting a Feb. 28, 2027, deadline for a construction report. FERC also told PG&E to “submit your plans for repair work at least 60 days prior to start of construction for review and authorization.”
PG&E filed its work plan Oct. 6. The letter, signed by Robert Ellis, the utility’s deputy chief dam safety engineer, describes “maintenance repairs on the downstream toe void of the embankment.” The toe of a dam is where its downstream face meets the ground. The letter says, “PG&E plans to start the project by November 20, 2026, and respectfully requests FERC authorization to proceed.” Nov. 20 is 45 days after the filing.
FERC’s public docket showed no response to the request as of Oct. 11.
State rates the dam satisfactory
The Cape Horn structure is a 283-foot concrete section, 63 feet at its highest, joined to a 237-foot earthfill embankment with a concrete core, according to FERC’s description of the project. FERC rates the dam’s hazard potential as high. Under federal dam-safety guidelines, that rating reflects the harm that could follow downstream if a dam failed, not the dam’s current condition.
The state Division of Safety of Dams, which got copies of FERC’s letters, lists the dam as “Van Arsdale” and rates its condition “Satisfactory” in its October 2025 list. The state rates Scott Dam, which forms Lake Pillsbury in Lake County, “Fair.”
FERC inspected the project again Aug. 5 and 6, 2026. Its Aug. 14 letter said the inspection produced no new action items.
In an Aug. 24 letter, FERC noted that PG&E narrowed or closed the Cape Horn fish ladder several times last winter, mostly during high water, starting in December. Salmon and steelhead resumed migrating through the ladder after it reopened, the letter said.
Removal still under review
The repair is separate from the plan to take the dam out. PG&E applied July 25, 2025, to surrender its license for the Potter Valley Project and remove Scott and Cape Horn dams. It would keep only the parts of Cape Horn needed for a new diversion, the New Eel-Russian Facility, which would allow Eel River water to keep flowing to the Russian River. FERC’s public comment period on the scope of its environmental review closed Sept. 18. PG&E’s proposed plans to reduce the project’s environmental effects are due by July 25, 2027.
PG&E also asked FERC on Sept. 2 for permission to drill near Cape Horn this fall to “evaluate the Cape Horn Dam embankment and foundation materials in support of the Potter Valley decommissioning.” PG&E plans to remove the dam.
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