Since 1922 most of the flow of the Fall River has been diverted at the Pacific Gas and Electric Pit No. 1 Forebay, located just upstream from Fall River Mills (Federal Energy Regulatory Committee Project No. 2687).
Flow is diverted via tunnel to Pit 1 Powerhouse on the Pit River. Generating an average of 60 megawatts of energy annually, the Pit 1 powerhouse operates as a peaking plant to help meet PG&E’s daily load swings. The project can divert from 200 to 2,028 cubic feet per second (cfs) of water, while maintaining surface water levels in the Fall River upstream of the diversion dam that generally reflect natural seasonal levels in the river. The forebay is 12 to 15 feet deep and fluctuates about 3 feet per day in response to project operations. Under the former license, there are no minimum flow release requirements from the forebay into the bypassed reach of the Fall and Pit Rivers or from the powerhouse into the Pit River.
There are two intake facilities to the Pit 1 intake canal and tunnel: intake No. 1 diverts water from the Fall River upstream of the diversion dam, and intake No. 2 diverts water from the forebay. The intakes open into two, short canal sections that converge into one common canal leading to a 10,076 foot long concrete lined tunnel. Most of the tunnel is horseshoe-shaped, 14 feet high by 13 feet wide. The tunnel terminates at a 60-foot-diamete, concrete-lined surge chamber with a spill channel. Two, 1,372 foot long penstocks, varying from 10 feet 9 inches at the upper end to 8 feet in diameter at the lower end, deliver water to the powerhouse, located on the Pit River about 7 miles downstream from the confluence of the Fall River and the Pit River. The powerhouse contains two, vertical-shaft, Francis-type turbines with a dependable capacity of 65.5 MW. Water flowing from the powerhouse is discharged through a 1,150 foot long tailrace channel. There are no transmission lines associated with the project. The switchyard is the point of junction with PG&E’s primary transmission system.