Lewiston Powerplant is operated in conjunction with the spillway gates to maintain the minimum flow in the Trinity River downstream of the dam. The turbine is normally set at maximum output with the spillway gates adjusted to regulate river flow. It provides power to the adjacent fish hatchery. Lewiston Dam is a zoned earthfill structure 91 feet high and 25 feet wide at the crest. The crest is 754 feet long.
Transmission lines were constructed and operated by the Bureau of Reclamation until October 1, 1977, when they were transferred to the Western Area Power Administration, Department of Energy. The powerplant originally served hatchery loads and station service requirements for Trinity and Judge Francis Carr Powerplants. Administration of the contract was transferred to Western in 1977 and the interconnection contract with Pacific Gas and Electric was canceled in 1989. A new interconnection contract was signed in 1990.
Lewiston maintains and regulates river releases. Energy in excess of hatchery loads is sold to Pacific Gas and Electric at 15 mills per kilowatt-hour. The turbine capacity is exceeded by the Trinity River minimum flow. There are options to increase the capacity at Lewiston that are not economical at the current price of power.