When California needs extra power, it generally imports hydropower from the State of Washington. This year though, a bit ominously, not only is there a California drought, the Pacific Northwest is also short on water. This means less hydropower for everyone.
Since 1989, hydroelectric dams have accounted for varying portions of electricity generated within California, from 11% in 1992 (reflecting a low water year) to a high of 28% in 1995 (a high water year).
Absent output from in-state hydroelectric resources, CAISO [California Independent System Operator] has tended to import more power from neighboring regions as well as increase output from thermal sources of generation. Much of the imported power comes from hydroelectric dams located in the Pacific Northwest, which is also experiencing low water supply.