California officials are courting a "very significant risk" if a damaged spillway on the nation's tallest dam is not operational by the next rainy season, and the state's plan leaves no time for any delays, a team of safety experts has warned in a report obtained Wednesday by The Associated Press.

Crews working to repair a crippled spillway on the Lake Oroville dam will be racing the clock to have the structure in good enough shape to be used for flood control 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 to operate 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 said.

The report does not explain the nature of the risk. However, officials with the state Department of Water Resources, which operates Oroville Dam and Lake Oroville -- the nation's tallest dam and California's second-largest reservoir -- fear that a huge rupture in the main spillway could expand and cripple a set of giant flood gates that control releases of water from the lake.

The independent consultants were selected by the state last month at the request of at the request of the Federal Energy Regulatory Commission.

Department of Water Resources spokeswoman Maggie Macias did not immediately return a call for comment Wednesday.

On Feb. 12, authorities ordered the evacuation of 188,000 people in three counties downstream of the dam after a chain of events that began with major damage detected on the main spillway just as a series of warm and very wet winter storms rolled across the Feather river watershed. Dam managers limited releases down the spillway, and within days, Lake Oroville surged uncontrolled over the top of an emergency weir and down an adjacent hillside. The resulting erosion raised concerns that the emergency structure would collapse and send a wall of water down the Feather River into Oroville and other communities.

The experts called it "absolutely critical" that the dam's state operators avoid another overflow on the emergency spillway. 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.