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The final monthly snowpack reading of this winter — 190 percent of the historic average — gives California its largest May 1 statewide Sierra Nevada snow level since 1998, when a major El Niño winter that year left the mountains with 201 percent of normal.
The Sierra Nevada snow is a key part of California’s water supply, providing roughly one-third of the state’s water. As the snow melts through every summer, it fills rivers and reservoirs, providing water for farms, cities and fish.
This year, it also poses a later-than-normal risk of flooding in some areas. Due to hot weather this week, starting Wednesday morning and peaking early Friday morning, the Merced River in Yosemite Valley — where temperatures are forecast to hit 84 degrees Wednesday — could overflow its banks.
The river’s flood stage is 10 feet. It is expected to exceed that height shortly after midnight Wednesday, peaking at 11 feet around 2 a.m. on Friday. Such a level, while inconvenient, would not cause major flooding in Yosemite Valley.
But it does serve as a reminder of the risk ahead. With so much snow in the high country, state and local water officials will be watching carefully and adjusting reservoir levels and taking other precautions in case there is a sustained period of hotter-than-normal weather.
“It will cool down over the weekend, but I think you’ll continue to see a flood threat on and off for a few more weeks,” said Dave Rizzardo, chief of the snow surveys section for the state Department of Water Resources. “There should be nice waterfalls in Yosemite for quite some time.”
The Truckee River, which is surging as the snow around Lake Tahoe begins to melt, also is forecast to slightly exceed its flood stage of 4.5 feet this week, peaking at 5.2 feet early Friday morning.
No Bay Area rivers are forecast to flood.
“There are a lot of people skiing with T-shirts and Hawaiian shirts out there today,” said Squaw Valley spokesman Sam Kieckhefer. “I haven’t seen any bikinis yet, but when we get our hot tub dug out of the snow in late May, we’ll see some.”
How much frozen water overall is stored up in the mountains?
On Monday, analysts at the state Department of Water Resources concluded that the “snow water equivalent” of the snow now burying the Sierra is 27.8 inches. In other words, more than two feet of water, spread out across the 400 miles of alpine country from Lassen County in the north to Tehachapi Pass in Kern County, is waiting to melt — nearly twice as much as the historic May 1 average of 14.6 inches.
Gov. Jerry Brown ended the state’s emergency drought declaration last month amid full reservoirs, floods and record rainfall levels in some areas, and nearly every community has lifted water restrictions. Still state officials urged Californians on Monday not to waste water in the months ahead.
“California’s cities and farms can expect good water supplies this summer,” said Bill Croyle, acting director of the state Department of Water Resources. “But this ample snowpack should not wash away memories of the intense drought of 2012-2016. California’s precipitation is the most variable in the nation, and we cannot afford to stop conserving water.”
Nearly all of California’s largest reservoirs are at above normal levels. The largest, Shasta Lake near Redding, is 94 percent full, or 109 percent of average for May 1. And Lake Oroville in Butte County, where contractors are rushing to repair a spillway that partially collapsed during heavy rains earlier this year, is 74 percent full, or 91 percent of average for this date.
Similarly, rainfall totals have been impressive.
San Jose has received 18.4 inches of rain since Oct. 1, or 123 percent of its historic average over that time. San Francisco has received 32.24 inches, or 143 percent. And Oakland is at 145 percent of normal, with 28.45 inches. Cities in the Central Valley and Southern California have had a similarly wet year. Los Angeles has received 18.67 inches, or 131 percent of average; San Diego 11.71 inches or 118 percent; and Fresno 17.08 inches, or 160 percent.