The historic drought that’s choked off rivers and reservoirs from the Rocky Mountains to the California coast is threatening to strain power grids this summer, raising the specter of blackouts and forcing the region to rely on more fossil fuels.

Many reservoirs that should be brimming with spring snowmelt show bathtub rings of dry dirt instead, including the largest one in the U.S., Lake Mead, which fell this week to a record low. Hydropower dams feeding off those reservoirs won’t be able to pump out as much electricity as they should, if they keep operating at all.