The Oroville–Thermalito Complex is a group of reservoirs, structures, and facilities located in and around the city of Oroville in Butte County, California.

The California Office of Environmental Health Hazard Assessment (OEHHA) has developed a safe eating advisory for fish caught in the Thermalito Forebay and Afterbay based on levels of mercury or PCBs found in local species.

The plant can also pump water back into the lake to be reused for power generation at Hyatt Powerplant when needed.

Lake Oroville is the second largest reservoir in California,[3] and stores winter and spring runoff which is released into the Feather River at controlled intervals to meet the Project's needs.

The Lake is perhaps the most important of all the Oroville Complex facilities, being that it serves as the headwater of the entire State Water Project.

[4] Lake Oroville has a maximum operating storage of 3,537,580 acre-feet (4.36354×109 m3), which, for purposes of scale, is equal to over 1.153 trillion gallons of water.

Beneath the dam a giant cavern almost as large as the State Capitol Building has been hollowed out to house six power generation units.

Located in rock in the left abutment near the axis of Oroville Dam, the Edward Hyatt Powerplant is an underground, hydroelectric, pumping–generating facility.

The facility was named for Edward Hyatt, who was State Engineer (1927–1950) of the Division of Water Resources under the Department of Public Works.

The diversion pool also acts as a forebay when Hyatt Powerplant is pumping water back into Lake Oroville, as well as provides recreation opportunities.

[8][9] On Thanksgiving, November 22, 2012, there was a major fire originating in a plant lower level cable tray that forced immediate shutdown.

The Afterbay provides storage for the water required by the pumpback operation to Lake Oroville, helps regulate the power system, produces controlled flow in the Feather River downstream from the Oroville–Thermalito facilities, and provides recreation to the area.

It also serves as a warming basin for agricultural water delivery to the numerous rice and grain fields just west of the Afterbay.

Thermalito Afterbay Dam, at 42,000 feet (13,000 m) long, is the longest crest in the California State Water Project system.

The Feather River Fish Hatchery was built to compensate for spawning grounds lost to returning salmon and Steelhead trout with the construction of Oroville Dam in the early 1960s.

Salmon and steelhead raised at the hatchery are transported in oxygenated, temperature-controlled tanks and released in the Feather and Sacramento Rivers, Lake Oroville, and/or in the Delta near the San Francisco Bay area.