Warming springtime temperatures in California are expected to accelerate melting of the state’s record snowpack, sending water surging down a major river in Yosemite National Park that could overflow its banks, officials said Monday.

Reservoirs downstream from the Sierra Nevada have been lowered in anticipation of the heavier-than-normal runoff, said reservoirs managers. The current snowpack is double its normal size.

The snowmelt flows downhill during warm months into reservoirs and canals, which supply one-third of the water used by residents of the most populous U.S. state. It also irrigates crops in the nation’s most productive farming state.

Forecasters warned that they expect the Merced River within Yosemite will hit flood stage in the middle of the week.

Nearly twice as much snow as normal blankets the 400-mile (644-kilometer) Sierra Nevada, a big contrast compared two years ago at the height of California’s drought when barely any measureable snow remained, state water managers said.

“What goes up must come down,” said Doug Carlson, a spokesman for the California Department of Water Resources. “The snow that’s sitting on the mountains will come down.”

The California Cooperative Snow Surveys Program on Monday was measuring how much water the snow holds, which should give officials an indication of how much water will be released by the snowpack.

The snowy winter has been especially cruel to wildlife in the mountains — killing off bighorn sheep and lengthening hibernation periods for bears.

During California’s drought, the iconic Sierra Nevada bighorn sheep moved from lower elevations higher up into the mountains in search of food, said Jason Holley, a wildlife biologist for the California Department of Fish and Wildlife.

But the heavy snow may have killed 100 of the 600 or so bighorn, he said.

“They’ve triggered some avalanches,” Holley said. “Others go caught in areas with no natural food.”

The snowdrifts have also kept many bears hibernating in the remote wilderness inside their dens one month longer than normal because food is still scarce, Holley said.

Hikers heading to the mountains are sure to find damaged roads leading to prized campgrounds that may not be repaired until next year, said Stanislaus National Forest officials.

In Yosemite National Park, rangers warned that visitors will need to be careful when they are near swift-flowing rivers and waterfalls with much higher water flows than normal.

Inexperienced hikers heading into the mountains should be prepared for snow lasting longer than normal this spring and should hike with more experienced people or consider heading to coastal mountains not covered in snow, said Kathryn Phillips, director of the Sierra Club California

“Not only is it technically difficult, it is pretty uncomfortable,” Phillips said. “If you’ve never done it before, go with somebody who has.”