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California’s massive snowpack is driving fears of major farmland flooding

Clouds pass above the snow-covered Sierra Nevada in the wake of an atmospheric river event that brought heavy snow to higher elevations near Lone Pine, Calif. (Mario Tama/Getty Images)
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After a season of massive snowfall, snowpack in California’s southern Sierra Nevada is way above its normal level. It sits at a whopping 261 percent of its typical April 1 peak, and this week, it sailed past levels from the 1982-83 season, a benchmark year in the modern record for snow and for floods. As spring begins and temperatures warm, the bulk of that snow will start to melt — driving fears of serious flooding in the valley below.

Runoff from recent storms is already filling the region’s reservoirs, sending floodwaters into farms and communities downstream, with more rain and snow still on the way.

“It is important to understand that we are in uncharted territory,” said Jeffrey Mount, a senior fellow at the Public Policy Institute of California’s Water Policy Center, in an email. “The amount of water tied up in snow in the watershed is about twice the average amount of runoff in an entire year.”

Mount said the snowpack is the biggest “since we started recording snow water content” with automated sensors. While the Department of Water Resources said that 1969 holds the top spot for the southern Sierra in the longer-term record, Mount believes levels will soon exceed that year as well.

Experts say the 1982-83 season is an example of how snowmelt can translate into epic flooding.

That year “produced the most significant flooding in living memory,” Mount wrote in a recent blog post. More than 100,000 acres of farmland were submerged, re-creating a historic inland lake. Floodwaters halted farming for nearly two years in affected areas.

Some of California’s most prolific farmland lies in the San Joaquin Valley and the Tulare basin at the base of the southern Sierra — downstream of this year’s heavy snowpack. Fresno, Tulare and Kern counties are the top three agricultural counties in the United States, according to a report from the Public Policy Institute of California.

Tulare Lake, once the largest lake west of the Mississippi, is now mostly an irrigated dry lake bed, carved with canals that channel water in and out of the basin. But the lake has re-emerged during extremely wet years like 1983 and 1997.

“It is logical to assume that we will see something like that again, but it all depends on how fast the snowpack melts,” he said.

It will also depend on how many more atmospheric river storms drop rain both above and downstream of reservoirs.

Reservoirs filling rapidly

Four rivers flow into the lake basin, draining water from the southern Sierra Nevada: the Tule, the Kaweah, the Kings and the Kern. The reservoirs in this region are much smaller than their counterparts in the wetter northern Sierra Nevada.

Mount said that two of the reservoirs, Lake Success on the Tule River and Lake Kaweah on the Kaweah River, are already full due to recent storms and have limited capacity to take on more water to prevent flooding.

“These are relatively small dams that fill quickly during high flow events,” he said. “And the channel capacity downstream is not large, so flooding is not that surprising here.”

The city of Visalia in the San Joaquin Valley is bracing for possible flooding as water is released from Lake Kaweah this week. And several evacuation warnings and orders have been issued in Tulare County because of flood concerns along the Tule River.

Lake Isabella on the Kern River to the south is also filling quickly.

“The risk is really coming from the fact that those dams now have to essentially release all of the water that flows into them,” Daniel Swain, a climate scientist at UCLA, said in an online presentation on Friday. “Rather than buffering flows, they’ve reached their safety maximum.”

While flooding so far has been limited to agricultural areas, he noted that the risk is likely to grow to include more populated areas and that the state was shifting significant response resources to the region.

With a break in the weather late this week, Mount said that operators will try to release as much water downstream as they can to make room for the next floods.

Farmers in Corcoran, Calif., on March 13 drove their trucks into a breached levee to block floodwaters from damaging their orchards. (Video: John Farrell/The Washington Post)

But this spring’s snowmelt could dwarf reservoir capacity, especially if it occurs quickly or during heavy rain from any additional storms.

In fact, the combination of full reservoirs, a massive snowpack and an active storm track is a dangerous one for downstream communities, according to Deirdre Des Jardins, an independent water researcher and director of California Water Research.

“We’re going to have prolonged, sustained high flows this year,” she said in an interview.

For many Sierra reservoirs, she said, the maximum flood level was calculated in the mid-20th century and hasn’t been updated to keep pace with changes in climate.

High flows this spring will probably strain the state’s many aging levees, which tend to be most neglected in low-income communities and weakest in agricultural areas.

Des Jardins thinks that officials could be more forthcoming on the possibility of flooding, so the public will be ready for evacuation orders, should they be issued.

“I think there is a balance — you want to be calm, but you need to be telling people that an evacuation warning means ‘pack’ and it means you might be flooded,” she said. “There is such a thing as being a little too reassuring.”