'The Big Melt': California braces for flooding as heat wave takes aim at state

 The colossal amount of snow in California's mountains from a winter of ferocious storms has started to melt, and an incoming heat wave is reigniting flooding concerns across the state.

"The 'Big Melt' is now officially arriving," UCLA scientist Daniel Swain warned on Twitter a few days ago. "Flows on many rivers draining the central and southern Sierra will double or triple (with locally greater increases) as temperatures rise. Some rivers will exceed flood stage, and Tulare Basin flooding will worsen."

A huge snowpack from record-smashing snowfalls may be a blessing for a region plagued by drought. But experts have warned it's also a potential disaster if it melts all at once. 

The state's vast wilderness, including Yosemite, and some critical agricultural communities are at particular risk for flooding. Forecasters are less concerned about the state's massive coastal cities.

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The lingering effects from winter storms are already causing rising river and stream flow levels not seen in years, the California Department of Water Resources reported.

Hot weather is in the forecast for California

"We're going to see things warm up, and with that warmth will come more snowmelt," said Michael Anderson, the California state climatologist, at a media briefing earlier this week. 

How warm? With highs expected well above 90 degrees, "near-record maximum temperatures are expected in the San Joaquin Valley on Saturday," the National Weather Service in Hanford, California, said.

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The warmth won't only affect the valleys, it will also be unusually warm in the mountains, too, forecasters said, with highs expected mainly in the 60s and 70s.

This heat wave will be relatively short-lived, UCLA's Swain said, noting that temperatures will cool to near- or even below-average levels across California by early next week. With the cooler temperatures, however, will also come the chance for an outbreak of showers and thunderstorms, he said. Partly because of the rain threat, "I expect high river flows will continue next week, despite cooler temperatures," Swain said on his Weather West blog.

Yosemite National Park closures begin – and may continue

As the heat hits snow in the mountains, the water runs down through rivers towards the sea. Two of those rivers are the Tuolumne and the Merced, and both run through Yosemite National Park – which has led officials to announce much of the eastern part of the park will close on Friday night and not reopen before Wednesday at the earliest.

Already the Merced is at eight feet, with ten feet considered flood stage. Models predict the Merced could get as high as 12 to 13 feet on Saturday, said park spokesman Scott Geidman.

There's already water on some roadways and meadows are beginning to fill up. As the waters rise, buildings could be cut off, sewage systems knocked out and roadways impassable. 

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It's expected that the waters will recede by Wednesday but more hot weather could easily cause additional flooding in the coming months.

"With this huge snowpack, we're looking at snow melt into June and maybe even July," he said.

Tulare Lake reappears for the first time in years

Shifting wet and dry years have been the norm for California for time immemorial. Several things have changed – agriculture and towns have grown; and more intense weather events caused by climate change are occuring

Prior to the build-up of agriculture and towns, the great valleys of California routinely flooded during wet years.

Today billions of dollars in agricultural products can be lost to flooding and the Central Valley region is home to 2 million people and their homes. Now when the land returns to its natural flooding patterns, the stakes are high.

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An example is Tulare Lake. More than a century ago this enormous body of freshwater in central California was the largest west of the Mississippi River, far larger than Lake Tahoe. It would grow in winter as snowmelt streamed down from the mountains.

But over time, settlers dammed and diverted waterways to irrigate crops, and as of 1920 the lake has been dry. Now, Tulare Lake reappears only during the rainiest and snowiest years.

The lake reappeared following wet winters in 1983 and on a smaller scale in 1997, said Nicholas Pinter, associate director of the University of California, Davis Center for Watershed Sciences.

It's back with a vengeance this year, having overwhelmed the dams and dikes meant to contain it in March. More than 100 square miles are flooded and more expected. State officials worry it may not drain for a year or more given the sheer amount of water on the ground.

Letting floods recharge groundwater

People in California downstream of the state's rivers that flow from the Sierra Nevada can look to the now snow-covered peaks with awe and concern. Statewide, snowpack is at 237% of average, equivalent to 61 inches of water.

That is a lot of water hitting the state's parched rivers and streams – water some are trying to put back where it belongs, in deep underground aquifers with levels that have dropped over years of drought and agricultural pumping.

Near the tiny town of Helm, Don Cameron is actively flooding fields that would otherwise be growing tomatoes and onions at this time of year, part of a project he's been thinking about for 30 years to recharge the groundwater under the 9,000 acres he farms. 

The dry north fork of the Kings River is now raging, allowing Cameron to divert 70,000 gallons a minute into his fields, where it’s quickly soaking down deep underground.

“We’re just flooding the heck out of the fields,” the owner of Terranova Ranch said. “We want to rebuilt our water supply.”

More flooding to come

He and his neighbors are preparing for more flooding to come. “It’s supposed to be 97 degrees here on Sunday. That will trigger more snow melt, there’s no question,” he said.

While flooding is a concern in Helm and the surrounding area, large swaths of northern California and western Nevada, including much of the central and northern Sierra, are under flood watches and warnings, the weather service said.

The newly-drenched fields in Helm are attracting geese, birds and even a bald eagle, a reminder of how the landscape of California’s central valley has changed, said Cameron, who is president of the California State Board of Food and Agriculture.

As he looks out on the gushing, lapping, rippling water he says the sight is “amazing. You bring water and you bring life. You see what it would have looked like 100 years ago, when this was a flood plain before dams were built.”

Contributing: The Associated Press