Droughts are cutting into California’s hydropower. Here’s what that means for clean energy.

The droughts that swept across the western US in 2021 sparked wildfires and damaged crops. But the historic lack of water also had an impact on one of California’s key sources of renewable energy: hydropower.

Electricity generation from California hydropower plants was down 48% from the 10-year average, according to new data from the Energy Information Agency. And 2022 is looking even worse.

Hydropower is the world’s leading source of renewable energy, making up about 17% of electricity generation in 2020, but droughts in various regions are making it harder to rely on. As a low-carbon source of power, it’s essential in limiting emissions of carbon dioxide, especially because when a hydropower plant goes down, fossil fuels are usually used to make up the shortfall. 

Hydropower plants made up about 19% of electricity generation in California in 2019. Most are located in the northern part of the state, where reservoirs are fed by melting snowpack from the mountains. But droughts over the last two years have caused reservoirs to dry up. The second-largest one in the state, Lake Oroville, saw water levels drop so low in 2021 that the hydropower plant there was shut down for the first time in its history.

The lost power can’t easily be replaced with renewable sources that fluctuate during the day, like wind and solar. When California’s hydropower capacity dropped from 2019 to 2020, much of the difference was replaced by natural-gas generation and electricity imports from other states, according to data from the California Energy Commission.

Hydropower often comes under fire for its environmental impact, because dams disrupt ecosystems. In fact, California currently doesn’t count large hydropower plants in its renewable-power targets. But regardless of how it’s categorized, hydropower is a lower-emissions alternative to fossil fuels. 

During high-stress times on the grid, the reduced reliability of hydropower is already causing problems, says Brian Tarroja, an energy researcher at the University of California, Irvine. 

Last year, the Bootleg fire in neighboring Oregon affected several transmission lines in California at a time when soaring temperatures had increased electricity demand. Running hydropower plants at their drought-reduced capacity while ramping up natural-gas plants was barely enough to keep the power on

These difficulties are likely to continue, Tarroja says. Climate change is altering rainfall patterns and causing higher temperatures, even if overall precipitation stays constant. The effects are likely to challenge hydropower in the coming decades. 

Places with high levels of hydropower may need to start planning for the effects of climate change on power generation. That’s not just California: droughts in Brazil and China have also threatened hydropower capacity in recent years. 

There will be some natural variation from year to year, but reprieve isn’t likely to come soon. Compared with last year, reservoir levels right now are “considerably worse,” said Aleecia Gutierrez, deputy director of the California Energy Commission’s Energy Assessments Division, in an email. 

Other renewable energy sources could eventually provide more reliable power to the grid, bolstered by technologies like grid-scale battery installations. But for now, losses in hydropower will likely mean more electricity generation from fossil fuels, and more emissions.