Firefighters today are doing mop-up work at a fire that knocked a state-owned underground hydroelectric generating plant near Oroville out of commission and caused an estimated several million dollars in damage.

The Thanksgiving day fire destroyed the control room of the Thermalito Pumping/Generating Plant, which is located a few miles downstream of Oroville Dam on the Feather River. The facilities are operated by the California Department of Water Resources.

State fire officials said they do not yet know what caused the fire in the five-story plant - four stories of which are underground. The fire was reported at 7 a.m. Thursday, mainly on the second lowest level. No one was in the facility at the time, and no firefighting injuries have been reported. State water officials said the plant is typically run remotely by workers upstream at the Oroville Dam plant facility.

Firefighters initially descended into the underground building to fight the fire Thursday, but were forced to withdraw because of lack of visibility amid black smoke. Firefighters spent the afternoon pouring water in, however, and reported the flames were mainly knocked back by evening.

Crews reentered the building Friday morning and have viewed fire, water, heat and smoke damage on the three upper floors. The building is expected to be out of commission indefinitely, state officials said.

"They were able to get through all five floors," Calfire Captain Scott McLean said Friday mid-morning. "It is smoldering right now."

Department of Water Resources officials say they are unlikely to be allowed into the building for several days and will not have a damage assessment done for at least weeks.

"We won't even have a rough idea of what we will have to refurbish and replace for at least a month," said state Water Resources spokeswoman Nancy Vogel.

Vogel said the plant closure will not affect local or state water deliveries because water flowing from the dam has been diverted around the damaged plant. State officials said the loss of the plant's electricity generating capacity may require the state to buy more electricity on the open market to help pump water statewide.

"This plant is less than 5 percent of electricity for the system," Vogel said.

An estimated 40 firefighters and 10 engines worked the fire Thursday, including personnel from CalFire, Oroville, Chico, Tehama County, Sutter County, and Marysville fire departments.

The fire is the latest safety incident at the state's Oroville facilities. In July 2009, a giant steel bulkhead blew out deep inside Oroville dam, injuring five workers.

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