美国国家公共电台 NPR--California's epic snowpack is melting. Here's what to expect（在线收听）

California's epic snowpack is melting. Here's what to expect

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CORCORAN, Calif. — The waters from a long-dry lake, resurrected by epic rains earlier this year, already lap at the levee of this Central Valley town of 22,000 people. A hundred square miles of crops are drowning around it. But the flood that Corcoran City Manager Greg Gatzka is really worried about has yet to come.

That flood — frozen in a historic snowpack — is still sleeping, piled around Sequoia trunks, some 80 miles away. Unseasonably warm temperatures are starting to wake it up.

For Gatzka, warmer temperatures mean "the snowpack, the ominous thing that we can see on the horizon ... is coming our way," he said.

Four major rivers empty into the landlocked southern end of the Central Valley and the clay-packed bed of the Tulare Lake Basin. All start in the snow-packed Sierra Nevada mountains and end, eventually, in the fast-growing expanse of Tulare Lake — what used to be the largest freshwater lake west of the Mississippi River.

"You can look at a scene like this and think the worst is behind us, when in fact it's quite the contrary," said California Gov. Gavin Newsom during a recent tour of Corcoran, which sits on Tulare Lake's historic shores. "Every day we're seeing an incremental half-inch, inch of new water present itself in the basin."

He described the scene as "surreal."

To understand the scale of the flood threat Corcoran and other communities are facing, and to get a sense of how local, state and federal officials are preparing for it, we followed the path of one river: the Kaweah River, which empties into Tulare Lake and begins in Sequoia National Park.

"Feet of water" sit in the snowpack

The headwaters of the Kaweah River are found in the southern end of the Sierra Nevada, home to Mount Whitney, the tallest peak in the lower 48 states, and the largest trees on Earth.

"Giant snow, giant trees," said Eric Meyer, an ecologist at Kings Canyon and Sequoia National Parks, during a recent trip to a still-closed section of Sequoia. At 7,000 feet of elevation in a wooded basin along the Marble Fork of the Kaweah, the snow is indeed giant.

The visitor's center is buried, and all of the cabins are cloaked in snow.

"We've got snow twice the size of our Chevy Colorado in an area that last year had very little," Meyer said, parking the truck in a canyon of plowed snow. "And all of this has to melt and go somewhere."

In normal years, that spring melt is something to be celebrated. The Sierra Nevada snowpack plays a critical role in California's water system. It's a giant frozen aquifer that provides a steady supply of water to rivers, trees and people downstream as temperatures warm and the rains stop.

This year, some basins have snowpack measured at 450% of normal for this time of year, Meyer said. The snowpack for the broader region is more than three times the average.

"Across the landscape, you have — sitting in the snowpack — feet of water in some locations," he said.

Already, it's starting to melt.

On an overcast, foggy day in late April, streams of water cascaded down rocks and ran next to roads in Sequoia. Hotter temperatures in more recent days have triggered flood watches farther north in the Sierra Nevada.

The Kaweah River hasn't seen a major increase in flows from snowmelt. Peak runoff isn't expected until late May or June. Like most watersheds in the southern part of the mountain range though, its geology makes it susceptible to flooding. The watershed's path is one of the steepest in the country. As snow melts, water rushes downslope in gullies and draws, merging into bigger and bigger streams and creeks.

Some is absorbed by the trees and ground. Most makes its way into the Kaweah River.

The dam that slows the water down

During a series of rain-heavy atmospheric rivers in January, water flows on the Kaweah River hit record highs.

Debris from recent wildfires turned its waters frothy black. Riverside beaches in the unincorporated town of Three Rivers, a 30-minute drive from Sequoia National Park, were stripped bare. Statewide, the storms caused billions of dollars in estimated damage.

The first major piece of human-built infrastructure on the Kaweah River, a 60-year-old earthen dam built across a canyon near the valley floor, withstood the deluge.

"Is there still flooding downstream? Yes," said Ryan Watson, the U.S. Army Corps of Engineers' deputy operations project manager at Terminus Dam. "Would it be significantly worse if this structure wasn't here? Absolutely."

Terminus Dam, like others funded after devastating floods in the 1930s, was built to regulate the flows of the nation's up-again, down-again rivers. California, more than any other state, is prone to climatic extremes — a trend that climate scientists expect to grow more intense as temperatures warm.

In years like this, when rainwater or snowmelt rushes down the Kaweah watershed, the dam serves as a speed bump. Water pours in and pools behind the dam before being released downstream in managed spurts.

"We can't control the river. We're not as naive as to think we have dominance over Mother Nature," Watson said. "She's going to do what she's going to do as we're seeing right now."

This year, dam operators have already exceeded flow limits for some of the channels below the structure. Some failed.

Watson and dam operators up and down the Sierra Nevada continue to dump water downstream in preparation for the big melt, essentially emptying the bathtub before the faucets turn back on. A tall strip of chalk-white rock showing where water levels used to be rings the rock shores of Lake Kaweah — the reservoir that's filled behind the dam. Watson hoped they'd get lower.

The emptier the reservoir, the bigger the speed bump, and the less likely people downstream are to have high flows of water coming their way.

Where the water gets put to use

Mark Larsen is one of the tens of thousands of people living downstream from Lake Kaweah. As the general manager of the Kaweah Delta Water Conservation District, his job is to help manage the water coming out of the Terminus Dam.

A map on the wall of his office's conference room shows the Kaweah watershed as it was in 1885, before people controlled the river's flow. There's no dam. And no Lake Kaweah. The river snakes down from the mountains unabated and spills out into the valley floor, branching into little braids of creeks and sloughs before coalescing to the southwest in Tulare Lake.

The map next to it shows the watershed a hundred years later. The dam exists — and so does the lake, created by construction of the dam. Below the dam, many creeks have been straightened, turned into canals. Many of the wetlands no longer exist, replaced by farms.

"And we wonder why we have flood issues," Larsen said, dryly.

In most years, nearly all of the water that comes down the Kaweah watershed is used up. "Fully utilized," as Larsen put it, by dairy farms and the almond, pistachio and alfalfa growers he helps serve.

That process, playing out across the region's other major watersheds, has led to the desiccation of Tulare Lake and one of the largest wetland complexes in the western United States. Snowmelt and rainwater that used to pool in the southern Central Valley's closed basin — providing habitat for waterfowl, fish and insects — rarely makes it that far.

This year, between the atmospheric rivers and snowpack, Larsen said, "We have more water coming into our system than we've managed since 1955." That's more than farms or towns can take.

As a result, he said, much of it is being released downstream toward Tulare Lake and Corcoran.

Residents anxiously watch the levees

At the Kaweah River's terminus in the Tulare Lake Basin, more than 100 square miles of land is already flooded. George and Judy Mendes have been anxiously watching the waters rise. They live in the northeast corner of Corcoran, near the lake's historic shore, just a couple of hundred yards from the city's main protective levee.

"If it wasn't for this levee, I'd probably be living at the YMCA shelter right now," Judy said, driving alongside a flooded field in their family truck.

This has become a daily routine for the Mendes family. Most mornings, George grabs a cup of coffee and drives to various points along the city's 14.5-mile levee and the flooded farmland beyond to see how much has changed. In recent weeks, he's seen private and county construction crews piling the levee higher in preparation for the coming snowmelt.

Beyond, fields of alfalfa are drowning in stagnant water. Tulare County Supervisor Eddie Valero recently estimated the losses in his county at $40 million. A supervisor for neighboring Kings County, which encompasses more of the lakebed, said crop damage has already exceeded $100 million.

"It's just devastating for the people here," Judy Mendes said. They know families who have had to flee flooded homes and farmers who are hoping insurance will keep them afloat. Mendes has bought flood insurance for herself and two family members in recent months.

State and local water officials expect water to remain in Tulare Lake for as long as two years. That's what happened in 1983, the last time the lake saw a big blast of water.

City officials are confident the levee around Corcoran will hold when the thaw accelerates because of the work being done to raise and reinforce it. The Mendes family, which owns a water tank business, has made plans in case it doesn't. They've lined up places to stay and to take their equipment.

"We're going to get a heck of a grand finale with all of that snow," George Mendes said.

The question nobody has a sure answer to is when.