U.S. hydropower output varies dramatically from year to year

Hydropower production depends on water availability and can vary significantly from year to year. Annual year-to-year variance in hydropower generation affects the overall contribution of renewable energy to the U.S. generation mix. Other forms of renewable energy, mainly wind, now play an increasing role in determining the overall contribution of renewable energy to the U.S. power generation supply mix.

Annual weather and precipitation cycles affect hydropower production. The hydroelectric resource potential depends on a combination of rainwater draining directly into waterways and the level of accumulated snowpack in mountainous regions that eventually melts and becomes run-off. For this reason, hydroelectric generating capacity may be derated in long-term forecasts, meaning that hydroelectric generating capacity has a discount factor applied to it, in the same fashion that wind generating capacity resources are derated. Because hydropower is comparatively inexpensive, fossil-fired generation and even wind generation can be curtailed when hydroelectric generation is high.

While the national hydropower output is often dominated by hydroelectric generating capacity in the Pacific Northwest, it is possible for other regions of the country to have opposite hydropower trends in the same year. For instance, in 2009 the Southeastern states had record-high reservoir levels and high levels of hydropower generation, while the Pacific Northwest received below-normal precipitation and, as a result, lower levels of hydropower generation.