Water levels in Folsom Lake and the American River this fall will drop to levels not seen in five years as California verges on another extended drought period.
The U.S. Bureau of Reclamation, which operates Folsom Dam, estimates the lake will fall to a storage level of 241,000 acre-feet by December. That is about one-fourth of total capacity.
The lake has not reached such lows since December 2008, the last extended drought period, when it fell to as low as 199,000 acre-feet.
Already, boat owners at Folsom Lake Marina face an Aug. 3 deadline to vacate their berths. The floating docks will be resting on the lake bed by then, when the storage level reaches 412,000 acre-feet, said the marina's manager, Ken Christensen.
It won't be long after that, he added, that a lakewide 5 mph speed limit will be imposed for safety. Though not unprecedented, these early restrictions on the lake are a convincing sign that dry times are at hand.
In an average water year, boats don't have to be hauled out of slips at the marina until Oct. 1.
"We are dropping about three-quarters of a foot a day," Christensen said. "It hurts business all around. We've got a lot of restaurants that depend on the public coming out and using the lake. We've seen a big drop in day use."
A shrunken lake also means less water to maintain flows in the American River. By October, the Bureau of Reclamation estimates, flows will drop to about 1,000 cubic feet per second, or about one-third of where they are today.
Similar concerns are developing at reservoirs across the state, including Shasta Lake. The bureau recently received approval from the state to modify protections for salmon in the Sacramento River to preserve as much cold water behind the dam as possible.
But Folsom Dam is in an especially precarious situation, because the American River watershed received even less snowfall last winter than the Shasta watershed.
In addition to serving local water agencies such as the San Juan Water District, which provides drinking water to Folsom, Fair Oaks and other communities, Folsom Dam also serves broader statewide demands for drinking and irrigation water. Folsom Lake water mixes with other diversions in the Sacramento-San Joaquin Delta, eventually serving millions of people from San Jose to San Diego.
Folsom is under additional pressure because it is the closest major dam to the Delta.
So it is often the first supply to be called upon when there is a need for fresh water to satisfy water-quality standards in the Delta for aquatic habitat.
The San Juan Water District has water rights in the American River, but also buys water from the Bureau of Reclamation at Folsom Dam. Its supply from the dam was cut back to 75 percent of normal this year due to the dry winter.
"At this point, we have been very active on (voluntary) water-efficiency measures, and we intend to continue with that," said Shauna Lorance, San Juan's general manager. "We also will be looking into using groundwater to offset some of the shortage."
Folsom Lake is just one component of the bureau's Central Valley Project, which also includes Shasta Dam, a water diversion system in the Delta, and about 250 water customers, most of them farmers. Bureau of Reclamation officials said they are doing their best to manage all these water demands.
"We are working with our partners to balance the needs of the water users that rely on Folsom Reservoir for their water supply as well as downstream commitments," said spokesman Pete Lucero.
The city of Sacramento depends entirely on its own water rights in the American and Sacramento rivers, and does not anticipate any shortages this year, said spokeswoman Jessica Hess.
But on Thursday the city issued its second voluntary "Spare the Water" alert of the summer, triggered whenever temperatures are expected to exceed 100 degrees for three days or more. The alert, which continues through today, encourages people to avoid nonessential water use, which saves water for other uses and also saves the city money in treatment and delivery costs.
"Our peaks are significantly less than what we've seen in the past," Hess said, thanks in part to ratepayers heeding the alerts. "We have seen a significant trend toward less per-capita water use over the past several years."
Other problems are emerging on the Klamath River. Last week, the bureau opened a public comment period on plans to release water from reservoirs on the Trinity River to help salmon runs downstream on the Klamath River. The run is expected to be large this fall, but without more water, another large-scale fish kill could occur like the one that left thousands of salmon dead in 2002.
The action is controversial because Trinity reservoirs also provide water to the Central Valley through a diversion tunnel. The estimated 62,000 acre-feet of water proposed to help salmon will mean that much less for Central Valley farmers, who oppose the move.
The state has not yet formally declared a drought, a move that could drive further conservation efforts. But officials are monitoring the situation, said Richard Stapler, a spokesman for the state Natural Resources Agency.
Last week, the State Water Resources Control Board notified senior water rights holders that it may act to curtail their water usage if conditions worsen. Such curtailments have not occurred since the drought of 1976-77, the most severe in modern times.
The notice was primarily aimed at farmers in the Sacramento Valley, who hold some of the oldest and largest water rights in the state. It came with a two-page list of conservation measures for "immediate consideration" to ease strain on water supplies.
Thad Bettner, general manager of the Glenn-Colusa Irrigation District, said the farmers he serves already have adopted most of those measures to cope with the dry year. The district also is working voluntarily with the Bureau of Reclamation on a plan to free up more water for Sacramento River salmon runs.
"I don't know, at this point in time, how much to make of it," Bettner said of the state notice. "We're going to have to really start looking into what we can do to move water around to get through these critical times."
The big worry is what happens in 2014. Another dry winter will mean even lower reservoirs, because they will be drained of any carry-over supply after this fall.
That could be a huge setback to the next generation of salmon in the American River and other streams, said Tom Gohring, executive director of the Sacramento Water Forum, a coalition of agencies working to improve flows in the American River.
"In 2014 we could end up in a situation, if Mother Nature decides to deliver a meager supply, with flows in the river at 250 cfs (cubic feet per second), which would mean you could walk across the river in most places," Gohring said. "It would be devastating, both from the standpoint of not having much habitat left, but also water temperatures would be lethal."
Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.