California flooding: 80% of Sierra Nevada snowpack hasn’t melted yet
Huge snowpack has begun to melt, but most of it — and flood risk in the San Joaquin Valley and Tulare Basin — remains
The huge Sierra Nevada snowpack, brought by a parade of atmospheric river storms that ended California’s drought this winter, isn’t a thing of the past now that warmer weather is here: 80% of the snow hasn’t melted yet.
On Monday, the state Department of Water Resources revealed that figure after conducting its final manual snow survey of the spring season at Phillips Station, a location in El Dorado County off Highway 50 near the Sierra-at-Tahoe ski resort.
Last year and the year before on May 1, they found bare ground at the site. In 2020, as California’s most recent drought was just beginning, they found a sparse 1 inch of quickly melting snow. On Monday, there was 6 feet of snow there.
Statewide, the Sierra Nevada snowpack was 254% of its historical average Monday, the second highest May 1 reading since 1950 when modern records began, behind only 1983, when it was 289%.
Why hasn’t the highly anticipated “Big Melt” happened yet? Conditions have been cooler than expected in recent weeks, with no major heat waves or warm “Pineapple Express” storms.
As a result, 80% of the statewide Sierra Nevada snowpack that was present on April 1 is still there. A month ago, that huge snowpack held an estimated 61.2 inches of “snow water equivalent” — the amount of water expected to run off when it all melts, DWR officials said. On Monday, however, 49.2 inches of snow water equivalent remained frozen across the Sierra.
That’s one of the reasons that flood experts are worried about significant flooding in the coming weeks and months in the Tulare Lake Basin and Southern San Joaquin Valley. Those low-lying areas between Fresno and Bakersfield sit downhill of the Southern Sierra, which received the most snow this winter.
“The significant majority of what was up there at its peak is still there, even as we head into May,” said Daniel Swain, a UCLA climate researcher. “This melt really is just getting started. I know that’s hard to believe.”
Swain said that it’s likely significant melting will begin in late May and early June as the weather continues to warm and summer approaches.
When all the snow does begin melting more rapidly, it isn’t expected to cause flooding in the Bay Area or Los Angeles.
But flood fears are very high in Kings, Fresno, Kern and Tulare counties where the 10-million-acre Tulare Lake once flourished as the largest lake west of the Mississippi River before being pumped dry over several decades in the late 1800s by farmers who diverted rivers that once filled it.
“While providing a significant boost to California’s water supplies, this year’s massive snowpack is posing continued flood risks in the San Joaquin Valley,” said DWR Director Karla Nemeth.
Currently, an estimated 74,000 acres of farmland in Kings County and Tulare County is now under roughly 3 feet of water, according to DWR, due to heavy rains in mid-March from atmospheric river storms. The low-lying areas have not been able to drain properly.
So far, as rivers like the Kern and Kaweah have begun to flow faster from the melting Sierra snowpack, reservoir operators have been able to stop new flooding by releasing water to lower reservoir levels, and capturing most of the incoming water.
But the fear is that when the weather becomes really hot — with temperatures over 100 degrees — for many days in a row, so much water from melting snow could flow into reservoirs that it will cause dam operators to release it in an uncontrolled way to prevent water from overtopping the dams. That could put more pressure on levees downstream, like the 15-mile levee that protects the town of Corcoran, home to 22,000 people and a state prison.
If a levee breaks, or a dam has problems, major flooding could occur.
“Some of these places are protected by earthen levees that are not designed to an extremely high standard,” Swain said. “And so there is concern regarding what happens there. They won’t be record-breaking inflows. But they will be sustained for weeks to come. And that is the problem.”
State officials from water agencies and the state Office of Emergency Services have been working with local police, planners and other officials in the area for the past month to map out possible evacuations, identify areas at risk like wastewater treatment plants, and stockpile sandbags and temporary flood retaining walls.
For the rest of the state, the giant snowpack means full reservoirs, a delayed fire season, and raging rivers across the Sierra that will pose risks to backpackers and weekend visitors in places like Yosemite National Park.
Only three other years — 1983, 1969 and 1952 — have had a snowpack at 200% of normal or larger on May 1.
“No matter how you look at the data, only a handful of years in the historical record compare to this year’s results,” said Sean de Guzman, manager of DWR’s Snow Surveys and Water Supply Forecasting Unit.
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