In its latest Short-Term Energy Outlook (STEO), the U.S. Energy Information Administration forecasts that electricity generation from U.S. hydropower plants will be 14% lower in 2021 than it was in 2020.

This is a result of “extreme and exceptional” drought conditions that have been affecting much of the western U.S., especially California and states in the Pacific Northwest, which are home to the majority of U.S. hydropower capacity. The Pacific Northwest’s Columbia River is the fourth-largest river in the U.S. by volume. Its watershed, the Columbia River Basin, covers large parts of Washington, Oregon, Idaho and Montana. In 2020, hydropower plants in these states generated 136 billion kWh of electricity, representing 54% of U.S. hydropower generation that year.

After dry conditions and a record-breaking heat wave affected large parts of the Columbia River Basin this summer, drought emergencies were declared in counties across Washington, Oregon and Idaho. The dry conditions have reduced reservoir storage levels in some Columbia River Basin states. According to the U.S. Department of Agriculture’s National Water and Climate Center (NWCC), reservoir storage in Montana and Washington is at or above average. However, as of the end of August 2021, reservoir storage in Oregon measured 17% of capacity, less than half its historical average capacity of 47%. Idaho reported reservoir storage at 34% of capacity, lower than its historical average capacity of 51%.

In March and April of 2021, hydropower generation in Washington and Oregon was 10% below the 10-year (2011 to 2020) range. Over the summer, hydropower generation in these states moved back within the 10-year range.

California contains 13% of the U.S.’ hydropower capacity. In 2020, hydropower plants in California produced 7% of the country’s hydro generation. However, the state is experiencing intense and widespread drought this year, which has reduced water supply and hydropower generation. The reservoir at Lake Oroville, the second-largest reservoir in California, hit a historic low of 35% in August 2021, prompting the Edward Hyatt Power Plant to go offline for the first time since 1967. So far this year, hydropower generation in California has been on the lower end of its 10-year range.

In the STEO, EIA forecast electricity generation for electricity market regions instead of state geographical boundaries. The latest STEO expects hydropower generation in the Northwest electricity region, which includes the Columbia River Basin and parts of other Rocky Mountain states, to total 120 billion kWh in 2021, a 12% decline from 2020. EIA expects hydropower generation in the California electricity region to be 49% lower in 2021 than in 2020, at 8.5 billion kWh.