The Lake Oroville dam poses a "very significant risk" if its damaged spillway is not operational by the next rainy season, experts warn, and the state's plan for the work leaves no time for any delays.
A report on the situation by a team of safety experts was obtained on Wednesday by The Associated Press.
Crews working to repair a crippled spillway at the dam will be racing the clock to have the spillway in good enough shape by next fall, according to the report prepared by an independent team of consultants and submitted to federal officials last week.
The crews hope to prevent a repeat, or worse, of dramatic events that led to nearly 200,000 people being evacuated last month.
Repair contracts will have to be awarded by June and workers will have to have the spillway in solid enough shape by Nov. 1, the experts warned.
"This is a very demanding schedule, as everyone recognizes. There seems to be no room anywhere to expand any part of the schedule," the five-member expert team said in the report for state and federal water and dam-safety officials. "A very significant risk would be incurred if the Gated Spillway is not operational by November 1."
The report does not specify what that means. However, officials with the state Department of Water Resources, which operates Lake Oroville — the site of the nation's tallest dam and California's second-largest lake — fear a huge rupture that opened in the main spillway could expand to cripple the flood gates that send out controlled releases of water and keep water from spilling over uncontrollably.
The independent consultants were selected by the state at the request of the Federal Energy Regulatory Commission.
Department of Water Resources spokeswoman Maggie Macias did not return a call for comment Wednesday. The experts on the panel either declined comment Wednesday or could not immediately be reached
On Feb. 12, authorities ordered the evacuation of 188,000 people in three counties downstream of the dam after surging releases of water tore away big chunks of the main spillway and then the dam's second, emergency spillway. At the time, authorities feared deterioration of the emergency spillway could send a wall of water from the lake downhill through surrounding towns.
The experts called it "absolutely critical" that the dam's state operators not use the faulty emergency spillway again.
The state should start work now redesigning a new emergency spillway for the 50-year-old dam, the consultants said.
The experts inspected the main spillway before delivering the recommendations to the state.
Water was even seeping from seemingly undamaged stretches of the main spillway, the experts said. Only 12 inches thick, the concrete spillway is heavily patched, at some places by clay stuffed into holes below the concrete.
"This calls into question whether the portions of the slab that appear undamaged by the failure should be replaced," the consultants said, raising the prospect of a much bigger long-term repair job.
Fully repairing the spillway will likely take two years, the consultants said. California still has at least a month left in the current, unusually wet rainy season. A record snowfall in the Sierra Nevada will send more and more runoff into Lake Oroville as weather warms.
State water officials plan to use the damaged main spillway sparingly to control the runoff, releasing water down it to try to ensure it doesn't spill out over the non-functional emergency spillway again.
Also Wednesday, the state Legislative Analyst's Office warned that tens of billions of dollars are needed for repairs and updates for aging dams, levees, wetlands and other projects in California's flood-management system.
Authorities have not provided a current estimate for the cost of repairs needed on the Oroville dam spillways.
© Copyright 2025 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.