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.

Hydroelectricity is relatively cheap, and a surge in production could help restrain rising utility bills in California, although the effect would probably be small. An increase in hydroelectric production should also curb California’s greenhouse gas emissions, by lessening the need for electricity from power plants that burn natural gas.

According to a closely watched Department of Water Resources index that tracks precipitation around Northern California dams, the region has seen 202 percent of its average annual rainfall. The agency’s measure of snowfall in the northern Sierra is also about 145 percent above average; that will help hydroelectricity producers, which rely on melting snowpack as a key source of water during warmer, drier months.

Back to Gallery How hydroelectric power has roared back in California 6 1 of 6 Photo: Paul Chinn, The Chronicle 2 of 6 Photo: Paul Chinn, The Chronicle 3 of 6 Photo: Paul Chinn, The Chronicle 4 of 6 Photo: Paul Chinn, The Chronicle 5 of 6 Photo: Paul Chinn, The Chronicle 6 of 6 Photo: Paul Chinn, The Chronicle











“We’ve suffered over the last five years enormously from a loss of hydropower because of the drought, so it’s great to have a good year,” said Peter Gleick, the co-founder and chief scientist at the Pacific Institute, a water think tank in Oakland. “Our reservoirs are full, the rivers are full, there’s a great snowpack ... and that’s great news.”

In 2015 — one of the state’s worst drought years — just 7 percent of the state’s total electricity output came from hydropower, according to the Energy Commission. The agency has not yet tabulated the state’s energy production figures for 2016.

“Right now, in our watershed, this is the wettest year on record, year-to-date,” said Ross Branch, a spokesman for the Placer County Water Authority. The authority’s records, he noted, stretch back 116 years.

“We have been running our hydro generators pretty much nonstop,” Branch said. “My understanding is that some of it’s coming so quickly, we can’t actually generate electricity from it.”

The Sacramento Municipal Utility District, which provides electricity to about 1.4 million people, is also witnessing a quantity of water not seen in decades.

“We’ve probably seen the wettest year since ’83. It’s been a very good water year so far for us,” said Chris Capra, a spokesman for the utility. “We generated more (hydroelectricity) in the first two months of the year than in all of 2015.”

The utility expects to generate 75 percent more hydropower this year than it initially budgeted.

But the torrents of rain have also caused extensive damage at some hydropower centers, straining their ability to make full use of the newly abundant water supply.

Branch said that debris and sediment produced by the recent spate of rainstorms have tended to settle near generators, clogging them and limiting their output. The generators, however, haven’t suffered any physical damage.

Jesse Saich, a spokesman for the El Dorado Irrigation District, said the water services provider sustained substantial damage to many of its flumes and canals. Repairs are under way in El Dorado County, Saich said, “but it’s going to take a couple of months to get it all fully repaired.”

“Our hydroelectric system relies on some flumes and canals that date back to the Gold Rush,” Saich said. “With the recent rains and the extreme precipitation, we’ve had an enormous amount of landslides.”

The Department of Water Resources brought all of the turbines at the Oroville Dam’s Hyatt Powerplant back online this month. They had been shut down to allow crews to remove debris related to the disastrous collapse of the dam’s principal spillway.

In an average year, 15 to 18 percent of California’s electricity generation comes from hydroelectric centers, said Gleick of the Pacific Institute. During the drought, he said, that number fell to less than 10 percent on average.

That deficit had to be made up from other sources, primarily the burning of natural gas, Gleick said, creating more greenhouse gas emissions.

“Over the five years of the drought, California’s greenhouse gas emissions from the power sector were 10 percent higher than they otherwise would have been, because of the loss of the loss of hydro and the fact that we had to burn more natural gas,” he said.

Natural gas is also a costlier energy source compared with hydroelectricity. Over the past five years, Californians doled out $2.4 billion more than they would have if hydropower output had remained at average levels, according to Gleick’s research.

But the anticipated rise in hydroelectricity output this year isn’t likely to make a tangible difference in consumers’ utility bills, at least not in the near future.

Pacific Gas and Electric Co. is one of the biggest players in hydroelectricity in the country. About 15 percent of the utility’s electricity comes from hydroelectric sources.

But the direct costs associated with generating or buying electricity represent only a portion of any PG&E customer’s bill. Most of the revenue PG&E collects from its customers covers expenses the utility incurs in actually delivering the electricity, including paying for the electricity grid. Those costs have to be approved by the California Public Utilities Commission, in three-year blocks.

“We don’t make money by energy sales — our profits are decoupled from that,” said PG&E spokesman Paul Moreno. “So what you pay in your electric bill, collectively, is what it costs to deliver that power. ... The cost of generation is just one of many factors that go into rate-making.”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa

No rain, no power

Percentage of California electricity produced in-state from hydroelectricity:

Year Pct 2010 16.6% 2011 21.2% 2012 13.8% 2013 12.1% 2014 8.3% 2015 7.1%