The US Energy Information Administration reports that in 2021, a historic drought that affected much of the western United States led to reduced water supply and, as a result, to lower hydropower generation in the Pacific Northwest and California. Generation at California’s hydropower plants last year was 48% below the 10-year average (2011–2020). In the Pacific Northwest, which consists of Washington, Oregon, Montana, and Idaho, hydropower generation was 14% below the 10-year average.
The western USA tends to be more susceptible to drought than the rest of the country because the level of precipitation is highly seasonal. Air masses coming from the Pacific Ocean carry moisture that falls as rain and snow on the western slopes of the Sierra Nevada range in California and in the Cascade range in the Pacific Northwest.
Rain and snowfall that accumulate during the winter and early spring months affect the region’s water supply. The snowpack in California’s Sierra Nevada mountain range on 1 April 1 last year was 41% below normal at the time of year when the snowpack holds the greatest amount of water. In contrast, snowpack levels in the Pacific Northwest last year were close to normal, according to the Northwestern River Forecast Centre. Extreme temperatures and heat waves also affected drought conditions in the western United States in 2021.
Shasta Lake and Oroville Lake, the two largest reservoirs in California, supply water to two of the state’s hydropower plants. Hydroelectric generation at Shasta power plant, the largest hydroelectric facility in the state, was 46% lower in 2021 compared with the 10-year average. Lake Oroville, the second-largest reservoir in California, hit record low levels in the summer of 2021, leading the adjacent Edward Hyatt hydropower plant to go offline for the first time. Electricity generation from Edward Hyatt in 2021 was 81% lower than the 10-year average.
In the Pacific Northwest, electricity generation at the Columbia River’s Grand Coulee power plant, the largest hydroelectric facility in the United States, was 12% below the 10-year average. Electricity generation at The Dalles hydropower plant, which is also on the Columbia River, was 14% lower in 2021 compared with its 10-year average.