Oroville-area firefighters mopped up Friday after a daylong fire knocked an underground hydroelectric generating plant out of commission and caused an estimated several million dollars in damage.

The Thanksgiving Day fire heavily damaged 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. No one was in the State Water Project 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 portions of the building Thursday to fight the fire, 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 re-entered the building Friday and found fire, water, heat and smoke damage on the three upper floors.

"They were able to get through all five floors," Capt. Scott McLean of the California Department of Forestry and Fire Protection said Friday.

The fire was considered officially out as of 1:30 p.m. Friday, and smoke was being vented from the building.

The building is expected to be out of commission indefinitely, officials said. Department of Water Resources officials said they are unlikely to be allowed inside 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 could require the state to buy more electricity on the open market to help pay to 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 Cal Fire, Oroville, Chico, Tehama County, Sutter County, and Marysville fire departments.

The blaze 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.

© Copyright The Sacramento Bee. All rights reserved.

• Read more articles by Tony Bizjak

Order Reprint