According to a new report from the U.S. Energy Information Administration (EIA), hydroelectric generation in the western United States (which the EIA identifies as the Pacific Contiguous and the Mountain Census Divisions) increased significantly in the 2021–22 water year relative to the 20-year low it experienced during the 2020–21 water year.

During the 2021–22 water year, western U.S. hydroelectric generation stood at 161.6 million megawatthours (MWh), a 12.9 percent increase (18.5 million MWh) relative to the 2020–21 water year (143.1 million MWh).

“Western U.S. hydroelectric generation can be quite variable because it is heavily dependent on rain and snowpack patterns,” said the EIA. Between the 2001–02 and 2019–21 water years, western U.S. hydroelectric ranged from a low of 152.4 million MWh (2014–15 water year) to 217.9 million MWh (2010–11 water year).

The recent increase in western U.S. hydroelectric generation was fueled by increases in precipitation in both the Pacific Northwest and California in the 2021–22 water year relative to the 2020–21 water year. In the Pacific Northwest, according to data from the National Oceanic and Atmospheric Administration’s (NOAA’s) Northwest River Forecast Center, precipitation was up in the Snake River, Upper and Middle Columbia Basins, Columbia River Main Stem, Western Washington basins, and Western Oregon basins.

In California, according to the California Department of Water Resources, precipitation from the Northern Sierra 8-Station Index increased from 24 inches in the 2020–21 water year to 43 inches in the 2021–22 water year, while precipitation for the San Joaquin 5-Station Index increased from 18.8 inches to 25.3 inches during the same time frame.

However, the EIA added, even at the higher precipitation levels in the 2022–21 water year, many basins in the Pacific Northwest and California were still below normal levels.

The increase in western U.S. hydroelectric generation was driven by a 17.4 percent increase in hydroelectric generation in the Pacific Contiguous Census Division, rising from 112.9 million MWh in the 2020–21 water year to 132.6 million MWh in 2021–22. All three states in the Pacific Contiguous Division had increases: Washington (17.4 percent, or 19.7 million MWh), Oregon (17.3 percent, or 5.3 million MWh) and California (14.6 percent, or 2.2 million MWh).

The Mountain Census Division, however, fell by 4 percent, decreasing from 30.3 million MWh in the 2020–21 water year to 29 million MWh in 2021–22. Although the two largest state generators, Montana (10 million MWh) and Idaho (8.6 million MWh), were essentially flat year over year, other states had decreases. Arizona’s generation decreased by 9.4 percent, and lower generation in other Mountain states, most notably Nevada, where generation fell by 23.5 percent, helped push total Mountain Division hydroelectric generation lower in the 2021–22 water year. Well-below-normal flow rates in the Lower Colorado River have also reduced hydroelectric generation in Arizona and Nevada.

“These regional differences are evident at the plant level,” said the EIA. In water year 2021–22, hydroelectric generation expanded by 19 percent (3.4 million MWh) for the Grand Coulee Dam in Washington, the largest hydroelectric generator in the Pacific Northwest, while generation fell by 10.0 percent (0.4 million MWh) for the Hoover Dam on the Arizona/Nevada border, the largest hydroelectric generator on the Lower Colorado River Basin.