ccording to a study by researchers at the University of California, Davis, the warmer and drier climate that is projected in computer models could reduce output from California's hydroelectric dams between 1,000 and 2,000 feet in elevation by about 20% by 2050.
How can small changes in temperature matter this much? Well, sometimes it doesn't take much to make snow turn into rain, and since snow acts as a buffer (either it stays in the mountains for a while, or it melts relatively slowly), the pattern of production is different than with rain that runs downhill immediately. It can also problematic if there's more rain, but in shorter, more intense bursts. Hydro reservoirs have a limited capacity, and it's better to have water come in over a longer period than all at once.
It also matters if the climate becomes drier and there's just less precipitation (which can happen locally, even if globally there's more precipitation). This isn't just in the future. It's already happening, according to some experts: "California, Oregon and Washington -- the source of nearly 50 percent of hydropower in the United States, according to the Pew Climate Center -- are already experiencing changes in snowfall, snowmelt and runoff consistent with climate change [...] ".