Shasta Dam, more than 600 feet tall and gatekeeper of the largest man-made lake in California, was designed to perform two crucial functions: store water and generate power.
And for decades, the huge concrete dam has channeled water to cities and farms while generating up enough electricity to power more than 532,000 homes.
But after four years of drought, the reservoir is drained to 50 percent of capacity, cutting the dam's power production by about a third, according to federal reclamation officials.
The story is the same at many dams across California, where electricity production at some is expected to be less than 20 percent of normal because of low water levels.
The shortfall shouldn't cause brownouts, officials said, because California relies on dams for power far less than it did in past decades, due in part to the emergence of solar and wind energy.
But it does come at a price.
Hydropower, even with its diminished profile, is important to California's energy mix as a quick, reliable and inexpensive source of electricity — a buffer during times of peak demand.
A reduced supply from dams forces the grid operator to turn to more expensive sources of power, such as natural gas.
"Consumers have paid more than a billion dollars more for electricity than they otherwise would've. And our greenhouse gas emissions are higher than they would otherwise have been," said Peter Gleick, president of the Pacific Institute, an environmental study group in Oakland. "Over the last three years, and continuing this year, the costs are going to continue."
In the 1950s, hydropower supplied almost 60 percent of the state's electricity. Now, it provides 14 percent to 19 percent in a normal year, and even less during a drought — accounting for about 8 percent of the state's total power last year. Renewable energy, on the other hand, provided more than 20 percent, according to the California Energy Commission.
Making up the difference from less hydropower has not been cheap. The cost to Californian could have been as high as $1.4 billion from 2012 through 2014, according to a report by the Pacific Institute.
Renewable energy, especially solar, helped make up for about 55 percent of the reduction in hydroelectricity in 2013 and 2014, state officials said. Natural-gas-fired power made up the rest.
Burning more natural gas to compensate for the reduced hydropower led to an 8 percent increase in carbon dioxide emissions from California power plants over three years, said Gleick, author of the Pacific Institute study. Hydropower produces little to no air pollution.
"If the drought continues," Gleick said, "if one of the impacts is a permanent reduction in hydropower, we need to ramp up other renewables even more than we are."
Experts said California had little choice but to diversify its power generation beyond dams.
Even in normal years, the dams have been producing basically the same amount of hydropower as they did decades ago. Huge population growth since the 1950s meant the need for more electricity. Instead of building more dams to supply the extra electricity, officials found other sources of energy.
"We've built on all of the good dam sites in California. We're not going to expand hydrogeneration almost anywhere in the West," Gleick said. "So in an expanding energy demand situation, hydro just becomes a smaller and smaller fraction of the overall system."
