By Rosanna Xia

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."

When hydropower plants were first built in California a century ago, they were sold as a way to handle two precious commodities: power and water. In 1909, while creating the Los Angeles Aqueduct, officials began building hydroelectric plants that helped power the construction equipment.

Hydropower is created when water passes through a turbine, which spins a generator and creates an electrical field. The power is sent to an electrical substation and enters the electric grid. Generating the power does not require extra water; after going through the turbines, usually in a power plant at a dam, the water returns to the river or canal system.

Unlike solar or wind-produced power, which depend on whether the sun comes out or the wind is blowing, hydropower is more controllable, officials said. At reservoirs, officials can turn a valve to increase the water flow whenever more electricity is needed. Energy suppliers rely on hydro as reserve power, and as a quick way to respond to surges in demand for electricity.

In drought years, it's a difficult balancing act because hydropower isn't always an immediate option.

"It's a much bigger operational challenge," said Colin Cushnie, Southern California Edison's vice president of energy procurement and management. "The hydro energy plays a very important role in maintaining system reliability, because of the flexibility it provides system operators."

Edison's hydropower plants are expected to produce less than one-fifth of its usual production this year, but this will not affect Edison's ability to keep the lights on for its 14 million customers, Cushnie said. Even in an average year, the hydropower Edison produces is only 5 percent of its total energy supply.

When hydropower is not as available, Edison has to rely more on natural-gas-fired power plants, which take longer to turn on and are more complicated to operate, Cushnie said. Wind and solar, and power purchased from other providers, have also helped Edison make up for the lack of hydropower.

Officials said costs will go down and there will be less reliance on natural gas to make up the difference as solar and wind power continue to grow. Wind and solar, accelerated by state mandates to focus on using renewable energy, have gone up more than 2.5 times in the past two years, said Robert Weisenmiller, chair of the California Energy Commission.

Wind and solar will also get cheaper and more reliable as more plants are installed, officials said.

But until this happens, Californians will have to stay drought-conscious, even when it comes to energy. Although it takes water to make electricity, it also takes electricity to move water from reservoirs to California's farms and showers.

"Saving water and saving energy are tied together," Weisenmiller said. "You can help save water by reducing energy use. It's really important that going forward, we use both energy and water wisely."

(c)2015 Los Angeles Times