Sierra snowpack is huge and melting fast – what that means for spring flooding

The wettest winter in Northern California history ended the drought and produced an abundant Sierra Nevada snowpack.

Now it’s starting to melt, and quickly.

As state officials completed the final snowpack survey of the season Monday, forecasters predicted high river flows throughout spring into midsummer.

The federal government’s California Nevada River Forecast Center said the undammed Merced River, which flows through the Yosemite Valley, could rise above flood stage this week. The flooding could create “nuisance” impacts such as closing access roads or campgrounds in the national park, said the forecast center’s hydrologist Alan Haynes.

The forecast center said about 5 percent of the snowpack has melted in the past few days, as measured in “snow water equivalent,” and potential trouble spots abound. Many of them are in the southern San Joaquin Valley, where levees have been under strain for months.

“The thing we’re watching out for … is the southern Sierra, where we have full reservoirs and in some cases a huge snowpack,” said Frank Gehrke of the state Department of Water Resources, which oversees the snow survey.

Speaking to reporters following the snow survey off Highway 50, he said DWR’s Airborne Snow Observatory, a joint venture with NASA’s Jet Propulsion Laboratory, will run additional flights over the San Joaquin rivershed through spring and early summer “so we can track the melt precisely.”

In Sacramento, county officials warned boaters and other recreational users about high flows on the American River this spring. Already Monday afternoon, a raft with five people aboard capsized when it hit a pedestrian bridge on the American River near Sunrise Boulevard. Three rafters made it to shore but two were left clinging to the bridge pilings until rescuers reached them.

Kim Nava, spokeswoman for Sacramento County Regional Parks, urged residents to avoid swift waters – or wear a life vest if they insist on going into the rivers. “Even the strongest swimmer can be pulled under,” she said.

Nava added that bicyclists and trail runners shouldn’t expect to see Discovery Park, submerged by the American River, to open any time soon.

Spring snowmelt as a rule doesn’t generate dramatic gushes of river water of the type seen during winter storms. But it lasts a lot longer, putting pressure on levees for weeks at a stretch, said Jay Lund of the UC Davis Center for Watershed Sciences.

“It comes out for a lot longer,” Lund said. “That load is going to be on there for a long time.”

Juggling the spring snowmelt will be particularly tricky at Lake Oroville, where DWR engineers are grappling with a fractured flood-control spillway that nearly created a catastrophe in February. DWR officials have said they expect to use the spillway at least once more this spring to reduce lake levels before closing the spillway gates for good and starting a $274 million repair that’s expected to last through next year. The lake was down to 835 feet Monday, well below the top.

At Phillips Station, a massive field near Echo Summit, the season’s final snow survey brought further evidence of winter’s bounty. Gehrke said he found 49.5 inches of snow and “snow water equivalent” of 27.8 inches. That amounts to 190 percent of average for May 1. The statewide snowpack average was 42.5 inches of snow water equivalent, or 196 percent of average.

The season’s final snow survey was conducted in decidedly springlike conditions, with temperatures in the 50s. Gehrke shed his customary bright-red winter coat and stood in shirtsleeves as he discussed the results with reporters. The veteran DWR employee noted that he had to cut the survey short because a portion of the meadow was already under water.

“Not unexpected,” Gehrke said. “It’s May 1; the sun is high in the sky.” He said the pace of snowmelt will accelerate in the coming weeks.

Down in the valley, the weather was considerably hotter, with Sacramento expecting high temperatures Monday in the upper 80s. Hannah Chandler, a meteorologist with the National Weather Service, said temperatures were expected to peak at 92 degrees by Wednesday in the region. Further north in the Sacramento Valley, the thermometer will hit 95, she said.

Chandler said temperatures will cool down later in the week. A small storm possible this weekend will bring temperatures down into the 70s.

With many reservoirs unusually full, Lund said some of the snowmelt will have to flow out of the Sacramento-San Joaquin Delta and into the ocean. But he said that probably won’t hurt the projected allocations for customers of the State Water Project and the federal government’s Central Valley Project, which deliver Northern California water to vast regions of the Central Valley, Bay Area and Southern California.

“A lot of it’s going to go out the Golden Gate,” Lund said. “This is the wettest year on record in the Sacramento Valley (and) it would be unrealistic to capture it all.”

DWR has said State Water Project contractors south of the Delta, including agencies serving millions of Southern Californians, can expect an 85 percent allocation this year. Central Valley Project customers have been told to expect 100 percent allocations for the first time since 2006.

DWR hydrologist Maury Roos said the Central Valley should expect high river levels through June on the Sacramento River and its tributaries – perhaps as late as July on the San Joaquin River side.

Dale Kasler: 916-321-1066, @dakasler

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