Hydroelectric power is produced when water turns a turbine connected to a generator. This water is stored behind a dam at elevation. Gravity causes water to drop toward a turbine propeller. The falling water turns the turbine, which produces power through the connected generator.

In California, hydroelectric power typically accounts for about 15 percent of the state’s annual power supply.

Benefits of hydroelectric power include:

• generating power when demand is high and energy is more valuable
• relative low cost
• near-zero emissions

Hydroelectric Power Overview

Hydroelectric facilities are divided into two categories: larger and smaller than 30 megawatts capacity. (A megawatt is roughly equal to the amount of electricity used by 750 homes at any given moment.)

The smaller power plants are considered producers of “green energy” or renewable energy.  These smaller plants are operated by utilities such as Pacific Gas & Electric, the East Bay Municipal Utility District or Southern California Edison.

Larger hydroelectric power plants such as those at Shasta Dam, Folsom Dam and Oroville Dam are operated by the federal Bureau of Reclamation and the California Department of Water Resources. [See also hydroelectric power and the State Water Project and Central Valley Project]. The Edward Hyatt Power Plant at Oroville Dam is the fourth largest energy producer of all hydroelectric facilities in California.

California has nearly 271 hydroelectric plants, mostly located at dams that also provide water supply, flood control and recreation. Together, these plants have a total dependable capacity of about 14,000 megawatts. Most dams in California are regulated by the Federal Energy Regulatory Commission.

The amount of electricity produced varies each year and is largely dependent on snow, storage and rainfall in upper watersheds–and the capacity of the reservoir where the power is produced. In 2019, hydropower-produced electricity totaled about 19 percent of California’s in-state generation portfolio, according to the California Energy Commission.

Hydroelectric Power and the Environment

Hydropower, a relatively pollution-free source of electricity, has helped lessen dependence on oil, gas and coal. Hydropower has also spurred agricultural and industrial productivity by providing millions of gallons of fresh water to the semi-arid Central Valley and Southern California.

But hydroelectric power can have a significant impact on the health of ecosystems.

Dams and reservoirs can alter stream flows, water temperature, turbidity (amount of sediment in the water) and oxygen content. This alteration of the natural water cycle can then affect native fish, which rely on spring floods to create wetlands habitat needed for sustaining young fish.

In response, there have been efforts to pay closer attention to natural cycles.

By altering peak flows, for example, dam releases can be made to more closely approximate the natural water flows of the river.

In some cases, removal of hydropower dams has occurred or is under consideration. In the Klamath Basin region along the California-Oregon border, the Klamath Hydroelectric Settlement Agreement allows for  removing four dams in the basin. 

Hydroelectric Power and Climate Change

California’s ability to meet hydropower demand in the face of rising temperatures could be problematic.

The anticipated reduction in runoff resulting from climate change means that less water will be available for hydroelectric generation. Hydropower production could decrease by as much as 30 percent as temperatures increase and precipitation decreases, according to the California Energy Commission.

Research published in 2020 in the journal Science said the West is in the midst of a historic megadrought caused by natural climate cycles and human-caused warming. According to the study, the years 2000 and 2018 bookended the second driest 19-year period in the West in the past 1,200 years.