Western US hydropower generation during the 2022-23 water year (ending 30 September) fell to the lowest level this century, raising questions over how much the region can rely on this longstanding clean energy source in an era of climate change.

Energy Information Administration (EIA) preliminary data shows output fell 11% to 141.6 million MWh from the previous water year, also among those with least generation in the past two decades.

EIA, the Department of Energy’s statistics arm, notes that hydropower generation can experience significant swings year to year because the amount of precipitation influences it.

“Precipitation mostly accumulates in the fall and winter months in the form of snow at higher elevations,” EIA said in its latest hydropower Electricity Data Browser, adding,“Snowpack accumulated during the winter serves as natural water storage that starts to melt in the springtime as temperatures gradually increase.

In May last year, a heat wave drove temperatures 30°F above normal, melting the snowpack rapidly. While May water flows were high, much of the water supply needed for generation during the summer months had melted earlier, particularly in Oregon and Washington.

California, in contrast, saw above normal snowpack last winter, which helped replenish reservoirs and double output of hydropower.

Still, the broader trend has been concerning with water levels below average in swaths of the west whose states include Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. These states produced most (60%) of the country’s hydroelectricity in 2022–23.

Low water has forced several major hydro facilities such as Glen Canyon Dam in Arizona and Hoover Dam in Nevada to reduce generation of electricity to 50-60% of rated capacity.

Supplementing and potentially replacing hydro in some cases as a baseload power source won’t be easy. It would likely require massive expansion of intermittent solar and wind, combined with battery storage, emerging technologies such as hydrogen, and greater energy efficiency.

The federal government owns about 50% of overall land area in western states, meaning Congress will have a large influence over the pace and timing of new renewable energy capacity in the region.

Some of those lands are off-limits for defence and environmental reasons, and it remains to be seen how much of what could be made available for clean generation will open for that purpose.

Hydropower has been about 8% of the nation’s installed power capacity for the past several decades, generating 6% of total US electricity in calendar year 2022, and 28% of power from renewables.