It may feel like spring in California, but this Memorial Day weekend, many rivers and creeks look like they're already trickling through summer.
Rafters and kayakers headed outdoors this weekend will find no room to float their boats on some waterways. Snowmelt runoff peaked a month early in some watersheds, leaving some of their streams too low for holiday water play.
The north fork of the American River near Auburn, for instance, is normally used for rafting and kayaking into early June. But by the end of April, its flows were already too low for boating.
"It is unusual for it to go so low this early," said Bill Center, a longtime California rafting guide and owner of the Camp Lotus rafting center on the south fork of the American River.
In the southern Sierra Nevada, the Kaweah River south of Fresno and the Kern River near Bakersfield are among other waterways flowing too low for boating, Center said.
Alan Haynes, a hydrologist at the California-Nevada River Forecast Center, a unit of the National Weather Service, said many streams are running at only about 10 percent of normal for this time of year.
"Because of the relatively light snowpack we had, and early melt-off, we're already looking at flows that are more typical of midsummer," Haynes said.
Tame rivers may not matter to a lot of people enjoying the outdoors this weekend. Forecasters predict unseasonably cool weather, and even rain, conditions unlikely to make people want to jump into the nearest stream.
The National Weather Service says high temperatures will reach only the 50s and 60s this weekend in the mountains and foothills, and the 70s in the Sacramento Valley.
Partly cloudy conditions are likely throughout the region Saturday and Sunday, with overcast skies and a chance of rain Monday.
The high in Sacramento on Monday is predicted to reach 71 degrees. The average for that date, in records reaching back to 1971, is 84 degrees.
For those who do go near creeks and rivers, experts advise caution, even with the lower-than-average flows. After all, the waterways are being fed with very cold snowmelt, which can immobilize a swimmer faster than many expect.
"Wear a life jacket, and be aware that it's going to be a little chilly this Memorial Day," Center said. "The key thing is to be careful."
He said tubing has been growing in popularity in portions of the American River's south fork, a waterway long popular for rafting and kayaking. Floating on an inner tube means being partially immersed in water at all times – not necessarily a good idea with cold May water temperatures.
"That's more of a July-August thing," Center said.
Many portions of the south fork are also too rough for tubing, Center said, especially for people with limited experience on the river.
Some rivers will still offer plenty of whitewater action this weekend and in the weeks ahead, including the south fork of the American River. That's because the recent federal relicensing of upstream dams operated by the Sacramento Municipal Utility District requires certain minimum flows to accommodate rafting and kayaking.
"The south fork is in great shape," Center said. "In fact, we're seeing this as being one of our better years, in part because of the predictable flows but also because it's been such an incredibly gorgeous spring."
The same goes for the Truckee River, a popular rafting, fishing and wading destination in the Lake Tahoe and Reno areas. Low snowmelt has left the river with below-average flows, but there's still enough for rafting.
Water releases from a small dam at Lake Tahoe regularly replenish the Truckee. Federal officials are required to maintain minimum flows, as long as the lake has water to release, to protect endangered fish species. Rafters and kayakers all the way to Reno and beyond get to enjoy that flow as a side benefit.
"We should do OK through the summer because of Lake Tahoe storage carryover from 2011," the last wet winter, said Chad Blanchard, watermaster for the federal office in Reno that oversees Tahoe water releases. "We didn't get a big peak that the kayak festival people (in Reno) would like to see. But because of storage, it hasn't looked that bad."
He said that snowmelt peaked in the Truckee River watershed on April 30, nearly a full month earlier than average. The river had its lowest peak natural flow – runoff not aided by reservoir storage – since 1988. Only one year since 1970 was worse, and that was 1977, a historically bad drought year.
The looming concern for Tahoe and other lakes throughout California is what happens in late summer and fall, when winter's meager snowmelt runoff is long gone.
By then, Blanchard said, Tahoe "is going to approach the rim," which means water releases into the Truckee River will no longer be possible.
Other reservoirs are likely to resemble puddles, relatively speaking, with vast exposed banks and restricted boating areas.
"Toward the end of the year, in the fall, we may start having trouble," Blanchard said. "But through the summer, we should be fine."
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Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.