Hydroelectric power is generated by the ability to turn falling water into electricity and in California accounts for about 15 percent of the state’s power supply annually.

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.

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 and Folsom 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].

California has nearly 400 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.

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 impact native fish, which for example, 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 this option.

Hydroelectric Power and Climate Change

Going forward, the California Energy Commission reports that California’s ability to meet hydropower demand in the face of rising temperatures could be problematic.

As a part of climate change, an anticipated reduction in runoff means that less water will be available for hydroelectric generation. According to the California Energy Commission, hydropower production could decrease by as much as 30 percent as temperatures increase and precipitation decreases.