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Frantic efforts to assess damage at major California dam as new storm system arrives

Damage to California's Oroville Dam concrete spillway this week has forced state engineers to consider alternatives to release water as new storms come into the Northern California region.

The damage also raises questions about the state's aging infrastructure at a time when President Donald Trump is talking about increasing U.S. spending on infrastructure. Oroville Dam, completed in the late 1960s, is California's second-largest reservoir.

The lower half of the spillway has concrete erosion, creating a gaping hole in a structure that is used for controlled releases of water. The reservoir's water levels were almost full on Thursday and the state was determining how it would do new water releases.

Speaking to reporters Thursday, a state official said there was no cost estimate for repair work.

The erosion was discovered Tuesday and initially raised safety fears for residents in Oroville that it could present a larger problem for the dam. However, officials from the state's Department of Water Resources insisted this week there's "no imminent threat" to public safety and said they are looking into "ways to bolster and protect the spillway."

Federal dam inspection officials were onsite Thursday and participating in the damage assessment. Officials were using drones to monitor the situation but stressed that the damage was not to the dam itself.

"The spillway is necessary to maintain reservoir operations, given the immediate forecast of continued rain for the next two days and also in preparation for the remaining runoff season," the DWR said Wednesday.

According to the National Weather Service, the Oroville area — about 70 miles north of Sacramento — is forecast to get 2 to 4 inches of rainfall in the next 48 hours. The same storm system is bringing precipitation to other parts of Northern California.

The spillway damage follows a series of storms to hit Northern California, producing rain and snow in the Sierra Nevada mountain range. Oroville Dam is located in the foothills of the western Sierras.

Engineers conducted two test runs with water flows on Wednesday, checking to see if the spillway can handle the strength of about 20,000 cubic feet per second of water.

As of Wednesday, the crater in the spillway was estimated to be a 200-foot-long strip. Officials said Thursday the cavity and erosion had grown substantially after the test releases.

"We're trying to determine if the spillway is not usable do we need to use an emergency spillway," Maggie Macias, a spokesperson for the DWR, said Thursday. She said using the Oroville emergency spillway would be the state's "last option but not our favorable option because it's a hillside and not a concrete structure."

The emergency spillway has never been used before.

Trees and rocks would likely get washed away if the state uses the emergency spillway, causing potential debris and mud problems downstream to a state fish hatchery. The emergency channel is designed as a final backstop to prevent water from flowing over Oroville Dam.

That said, state workers Thursday were clearing the hillside of vegetation, rocks and other possible debris in case the emergency spillway was needed to handle the additional water from rains. They also were relocating fish as a preventative measure.

Oroville has a capacity of 3.5 million acre-feet of water at around the 900-feet elevation.

The state website indicates the reservoir now holds about 3.1 million acre-feet of water. The state said Wednesday enough vacant space exists in the reservoir to capture the flow of the rains expected through Friday afternoon.