MENDOCINO CO., 11/9/26 — PG&E has a conditional OK to drill into Cape Horn Dam on the Eel River, one of two dams it wants to tear down. The holes will test the dam, the ground under it and the river sediment that has built up behind it, according to records filed with state and federal dam regulators.
Cape Horn Dam holds back Van Arsdale Reservoir. An intake 400 feet upstream of the dam takes in Eel River water. The water runs through the Potter Valley Project’s tunnels to the East Fork of the Russian River, according to the Federal Energy Regulatory Commission.
PG&E has asked the commission, known as FERC, to let it give up its license for the project and remove both Cape Horn and Scott dams. It would keep only the parts of Cape Horn needed for a new diversion, the New Eel-Russian Facility. In a May planning document, FERC staff wrote that removing the dams without that facility would end future diversions to the Russian River. It would also hurt PG&E’s duty to deliver up to 19,000 acre-feet of water to the Potter Valley Irrigation District.
In a Sept. 2 letter, PG&E told FERC the drilling is meant “to characterize reservoir sediments as well as to evaluate the Cape Horn Dam embankment and foundation materials in support of the Potter Valley decommissioning.”
The drilling plan itself isn’t public. PG&E filed it as critical energy infrastructure information, marked “DO NOT RELEASE.”
Questions about a wall
FERC’s San Francisco regional office answered Oct. 29. It told PG&E it could go ahead with the drilling if it answered FERC’s comments by Nov. 8.
Two of the holes are planned close to a concrete retaining wall at the dam. FERC asked PG&E to explain how it will monitor that wall while it drills. FERC’s letter lists surveys before and after, and “visual observation of existing seeping cracks or formation of new cracks.” It also asked PG&E to weigh the risks of drilling that close to the wall and to send a map of where those holes will go.
FERC also flagged a mismatch. Two other holes are planned to go about 30 feet deep, but the plan expects 20 to 60 feet of sediment behind the dam. FERC asked whether those holes should go past the sediment.
PG&E answered Nov. 5. Its letter to FERC says all four comments “have been satisfactorily addressed” and “no further action is necessary.” PG&E also sent a revised drilling plan. Its answers and the new plan are both marked “DO NOT RELEASE.”
The state’s sign-off
The state Division of Safety of Dams also regulates the dam, which it calls Van Arsdale. On Nov. 2 it filed a notice saying the drilling doesn’t need a full state environmental review. The notice calls the work “minor repairs and alterations to existing dams.” The division’s October 2025 list of dams rates Van Arsdale’s condition “Satisfactory.”
PG&E told FERC it planned to start drilling in fall 2026, or as soon as it had all its permits. It said it would tell FERC the drilling was done by Jan. 31, 2027.
The drilling is one of two jobs PG&E has lined up at Cape Horn this fall. In an Oct. 6 letter, PG&E told FERC it plans to start repairing a void at the base of the dam by Nov. 20.
FERC is still reviewing PG&E’s request to give up the license and remove the dams.