Will a ‘wall of water’ flow down from Sierra Nevada when snow melts? Experts weigh in

A record-breaking spring snowmelt is being forecast for the southern Sierra Nevada watersheds that feed rivers flowing into the Tulare Lake Basin, bringing with it the potential for “severe flooding” of farmland and communities in Kings and western Tulare counties.

Just how bad it gets, however, will largely depend not on the frequency and ferocity of the storms that have slammed central California so far in 2023 — already creating flood woes for communities across the Valley — but on how quickly the weather warms up moving forward.

Either way, “the staggering amount of snowpack (in the southern Sierra Nevada) and the very intense runoff conditions that we anticipate are going to create significant challenges with long-duration flooding in that part of the state,” said Karla Nemeth, director of the state Department of Water Resources, during a press conference Monday at a snow-survey site south of Lake Tahoe.

The nature of the problem “depends how quickly California warms up as to how quickly we will experience the snowmelt,” Nemeth added. “We are into those longer daylight hours which can trigger snowmelt at a much more rapid pace.”

Just how much water is sitting up there socked away in the many feet of high-elevation snow? Automated snow sensors at more than 260 sites across the state indicate that the snow water equivalent in the state’s snowpack is 237% of the April 1 average, said Sean de Guzman, manager of snow surveys and water supply forecasting for the Department of Water Resources.

This year’s snowpack statewide holds more water at the start of April than any year since the snow sensor system was deployed in the mid-1980s — and possibly more than any year in snow survey records dating back to 1910, de Guzman said. But from one watershed to another across the state, the snow-trapped water can vary greatly.

“We’re currently forecasting record-breaking spring snowmelt in the Tulare Lake region, which is ranging anywhere between 265% of average in the Kings River watershed,” de Guzman said, “upwards to an absurdly high 422% of average for a snowmelt runoff in the Kern River watershed.”

Will warming weather bring trouble this week?

After a cold but largely dry system on Monday brought high winds but little rain or snow to central California, a warmup in daytime high temperatures is forecast through early next week, reaching the high 70s or low 80s.

But a forecaster with the National Weather Service in Hanford said it will take even more warming to make a real dent in the massive snowpack.

“Just because it warms to the 80s in the Valley doesn’t mean a wall of water coming down the hill,” Bill South, a meteorologist in Hanford, told The Fresno Bee. “For a snowpack like we have this year, that’s so deep and so cold, it’s going to take a substantial warmup for an extended period of time.”

The modest warmup over the coming days, South added, “will warm the snowpack and nothing more.” That’s because sensors aboard satellites that pass over the region indicate that “the substantial snowpack has very low temperatures, even as low as zero,” he said.

“Until we get snow temperatures into the 30s, it’s not going to melt,” South said. “It will take temperatures reaching 90 or 95 (degrees) in the Valley, and it would have to get into the 70s at about 7,000 feet. That usually does not happen until May.”

Too much water for the region’s dams to contain

Climate scientist Daniel Swain from UCLA said there’s “no big huge enormous scary melt events on the horizon” that would accelerate imminent melting of the snowpack.

“I don’t see any big heat waves or any major storms that would cause them in the next week or so,” Swain reported in a recent video post, “but we’re going to be on high alert for those kinds of events really for the next month or two all the way through May.”

There are four dams on rivers that flow from the southern Sierra Nevada into the southern San Joaquin Valley that historically drained into the once-dry — and now refilling — Tulare Lake in Kings County.

At Pine Flat Dam on the Kings River in eastern Fresno County, about 696,000 acre-feet of water was stored on Monday in Pine Flat Reservoir. That’s down from a peak of almost 784,000 acre-feet on March 21, as the U.S. Army Corps of Engineers that operates the dam steadily releases water to make room for the coming snowmelt. The reservoir has a capacity of about 1 million acre-feet.

An acre-foot equals almost 326,000 gallons, and is the volume of water needed to cover one acre of land to a depth of one foot. An American football field — minus the end zones — is just over an acre in size.

Monday’s storage at other dams in the southern Sierra Nevada watershed, reported by the state Department of Water Resources, are:

• Lake Kaweah, behind Terminus Dam on the Kaweah River east of Visalia: 106,324 acre-feet, about 57% of the lake’s capacity of 185,000 acre-feet.

• Lake Success, behind Schafer Dam on the Tule River east of Porterville: 42,786 acre-feet, about 52% of the lake’s capacity of 82,300 acre-feet.

• Lake Isabella, behind Isabella Dam on the Kern River northeast of Bakersfield: 331,885 acre-feet, about 58% of the lake’s capacity of 568,000 acre-feet.

The problem is, there’s more water up in the mountains than the dams can hope to contain. On the Kings River alone, the Kings River Water Association reported Tuesday that state forecasters predict that snowmelt runoff from the river’s watershed could generate between 3.1 million and 3.6 million acre-feet of runoff.

A year ago, April snow surveys by the Kings River Water Association and Pacific Gas & Electric Co. reported an average of 29 inches of snow water equivalent across 22 courses measured. This year, the water content is more than double that, at 73.3 inches, or more than 263% of the historic April 1 average.

And that water will eventually be coming down the Kings River and the other rivers in the southern Sierra.

“The challenge is that we really haven’t seen much snowmelt yet,” Swain said. “There is more snow up there now than there has been at any point so far this season, a record amount.”

Between now and June, “it’s going to fill some of these smaller southern Sierra reservoirs multiple times over,” he said. “That means essentially those reservoir operators are going to have to release water continuously, potentially (at) high flows to maintain safety margins in these reservoirs and with these dams.”

When the snowmelt peaks, “they may just essentially have to let as much water out as is coming in,” he added. “Those dams may cease to function as flood control features of the flood protection system.

“That does not mean that the structures themselves are necessarily going to have structural problems,” Swain said, “but what it might well mean is that the operators of these dams (are) going to have to set outflow equal to inflow, meaning it’s essentially just whatever Mother Nature throws at us.”

In that circumstance, the dams cannot serve as buffers to large flows, “and there’s going to be severe flooding downstream of that,” he added.

Adding to problems in Kings and Tulare counties

That’s not good news for farms and communities that have already confronted floodwaters from a recent series of atmospheric river storms that drenched the region.

“The real challenge, as we move into spring and summer, is flooding, significant flooding, particularly in the Tulare Lake Basin,” Nemeth, the state water resources director, said Monday. “The department is very focused on the entire Central Valley including the Sacramento Valley, but certainly very focused on some of the challenges the communities in the Tulare Lake Basin will experience this year.”

Nemeth noted the irony of an overabundance of water after several years of severe drought conditions and limited water supplies for communities and agriculture in the San Joaquin Valley.

“We do have Californians that will continue to struggle with water supply, particularly those in the Central Valley and particularly those communities that are dependent on groundwater wells,” Nemeth said.

“It is possible that given the conditions in the Central Valley, particularly in the Tulare Lake Basin, that there will be simultaneously water supply challenges that come along with drought but also water supply challenges that come along with very, very significant flooding.”