LOS BANOS — In 1805 Spanish soldiers camped here in the oak-studded valleys. California’s Robin Hood, Joaquin Murrieta, hid out here during the Gold Rush. President John F. Kennedy made a visit in 1962.

There’s no question the history around San Luis Reservoir is colorful. But these days, the star attraction isn’t much to look at.

This vast inland sea along Highway 152 between Gilroy and Los Banos — the largest off-stream reservoir in the world — sits just 17 percent full.

The shoreline is a vast expanse of dried, cracked mud. Boat ramps end above the water’s edge. Hills show erosion lines where the lake’s surface once lapped in wetter years 168 feet higher than today.

This year, the reservoir was at it lowest level of any Aug. 1 since 1989. And back then, California was knee-deep in its last major drought, which lasted from 1987 to 1992. A record-dry spring this year and pumping restrictions at the Delta are to blame now.

The low level is making water officials nervous in Silicon Valley, which draws billions of gallons from the reservoir.

“It’s a concern for us every year, but more of a concern for us this year,” said Joan Maher, deputy operating officer for the Santa Clara Valley Water District, based in San Jose.

When the lake level drops, the water warms, which causes algae to grow.

And when the water is pumped out every day from the reservoir, through a 10-foot-wide pipe 42 miles to the Coyote Pumping Station in Morgan Hill near Anderson Reservoir, it requires lots of treatment. Even after that, it doesn’t seem quite right when it comes out of Silicon Valley taps.

“People have been calling, saying, ‘What’s happening with the water?'” Maher said. “The water is safe to drink, but this makes it taste and smell a little musty.”

The people most affected live in Saratoga, Cupertino, Campbell, Los Altos and other communities that receive drinking water from the district’s Rinconada treatment plant in Los Gatos. That’s the only one of the district’s three drinking water treatment plants that doesn’t have high-tech ozone treatment — it won’t for another three years or so. For now, the plant is having to clean its filters more regularly, and crews have increased the amount of granulated carbon they use to absorb odors by tenfold from normal years.

When full, San Luis holds 2 million acre-feet of water, enough to supply the needs of 10 million people for a year.

There are two reasons San Luis is so low now.

First is the dry spring. After a wet November and December, California experienced record-dry conditions starting in January.

The amount of snow and rain that fell in the northern Sierra between January and the beginning of April was the least since records were first kept in 1920. Most Bay Area cities had the driest spring in their history.

Less snow and rain meant less runoff. Still, other large reservoirs in Northern California have much more water now than San Luis. Lake Shasta, Oroville, Trinity and New Melones are all between 46 and 61 percent full.

Another reason San Luis has not filled as much, Maher noted, is because its water is pumped from the Delta, 75 miles to the north. And in recent years, federal court decisions have limited the time and scale of how much water could be pumped out of the Delta through the giant federal and state water projects that store water in San Luis for later use, not only by Silicon Valley, but by farmers and cities throughout the Central Valley and Southern California.

As Maher and many other water officials see it, one solution is Gov. Jerry Brown’s $24 billion plan to build two massive tunnels under the Delta to more easily ship water south.

Environmentalists say the tunnels will wreck the Delta’s fragile ecosystem and make it easier to ship water to subsidized corporate farmers. They argue the problem is that agriculture — which uses 80 percent of the water that people consume in California — simply takes too much from the Delta and has planted crops that rely on more water than the Delta can deliver consistently without killing salmon, smelt and other species that live there.

“If we weren’t using our precious water to grow almonds to ship to China, we’d have more for urban use,” said Barbara Barrigan-Parrilla, executive director of Restore the Delta, a Stockton group.

In the past week, water levels at San Luis Reservoir have crept up 3 feet. That’s because demand from farmers is waning in the later part of the growing season and some Delta pumping is occurring, said Tracy Pettit with the state Department of Water Resources.

The lake should keep filling, and conditions will improve, unless California has another dry winter. Then San Luis could fall even lower by next summer, dropping below the intake pipes that the Santa Clara Valley Water District uses.

The district won’t run out of water. It has a year’s supply of groundwater, 10 smaller local reservoirs and a recycled water plant coming online in the fall. But a nearly-empty San Luis could contribute to calls for water rationing next year.

“If we see continued dry hydrology, there is a concern that it could be low again,” Pettit said. “That’s one of the biggest uncertainties. We’re hoping for a wet winter.”

Paul Rogers covers environmental issues and resources. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN.