After slowing to a trickle during the past five years of punishing drought, hydroelectric power in California is poised to make a major comeback this spring and summer, thanks to the wet winter. Across Northern California, hydroelectricity producers say their reservoirs are brimming at levels not seen in decades. Together, their dams should produce as much as 21 percent of the state’s total electricity output this year, according to projections from the California Energy Commission. That would be the highest percentage for hydropower since 2011, according to the commission’s Energy Almanac. That was the last wet winter before the drought.