Could a record snowpack mean flooding in the Valley?

As the Central Valley braces for another atmospheric river-driven storm arriving early next week – bringing another 1.5 to 2 inches of rain starting late on Saturday and intensifying into the week – a potential catastrophe is accumulating in the Sierra. As the snowpack at elevation continues building well past historic high levels, the possibility of intense floods in the Valley grows ever more likely.

280 Billion Gallons

For the entire Sierra Nevada Range, the snowpack is at 212% of the average. The snow, however, isn’t distributed evenly. The southern Sierra to our east bears an outsize proportion, with the snowpack at 257% of normal on March 12. Experts at the Kaweah Delta Water Conservation District (KDWCD) put the total water stored in the snowpack at 700,000 acre feet.

That’s equivalent to more than 280 billion gallons of water.

As the snow and ice melts, more than one-quarter trillion gallons of water – equivalent to about 110 to 120 inches of rain – will enter the watershed for the Tulare Lake Basin. Depending on how fast it melts, the Valley could see widespread flooding as a result. It all depends on the weather staying cool.

“We’re hoping the heat lamp doesn’t come on,” said Bill Miller, operations project manager for the US Army Corps of Engineers (the Corps) at Lake Kaweah and Terminus Dam.

Reservoirs at Capacity

The coming snowmelt will filter into the four river systems that make up the Tulare Lake Basin Watershed: the Kings, Kaweah, Tule and Kern. All four of those systems have dams and reservoirs maintained by the Corps, and all four of them are already taking in more water than they’re putting out.

Early Thursday morning, Lake Kaweah reached capacity and a small flow began pouring through the Terminus Dam spillway. According to Visalia city officials, the overflow at Terminus increased the threat of flooding in the city. It prompted them to broadcast a flood warning:

“We could see localized flooding near our waterways,” they wrote. “This means that water could reach capacity in our own waterways and spill over at various locations.”

No evacuations are planned.

The threat of flooding downstream of Terminus Dam will likely rise as the coming storm again increases the inflow of water at Lake Kaweah to extreme levels. During the latest storm on Friday, the inflow to the lake reached 41,000 cubic feet per second (cfs). As of Thursday night, the inflow – at about 5,200 cfs – finally dropped below the outflow rate. The Corps increased outflow throughout the day on Thursday to finally reach maximum.

“Luckily right now, we are currently releasing 5,500 cfs, which is the capacity,” Miller said. “The overflow (at the spillway) is allowable. We’ve got to make room in the lake.”

Crews at the dam are performing regular inspections and maintenance – literally working around the clock – while coordinating with downstream water authorities, Miller said.

“We’re trying to maintain these flows and keep them out of Visalia,” he said.

There is a specific health risk involved with the water flowing out Kaweah Lake. Some of the composting toilets at the lake’s various recreation areas – and the sewage they contain – are now submerged.

Water System Already Stressed to Minor Breaks

The storm system coming next will be the 13th atmospheric river (AR)-driven storm to strike the state since November. While the storm will be driven onshore by a flow from the tropics, the storm itself is dropping down from the Gulf of Alaska, and it could again spread snow as far south as Los Angeles. It will also drop another 4 to 6 feet of snow in the southern Sierra and up to 2 inches of rain on the Valley floor. Forecast models show a 14th AR storm forming behind it that could make landfall late next week.

Whether the system of waterways in the KDWCD can handle that much more water remains to be seen.

“We’re pushing the system to the limits with what Mother Nature is handing us,” said Mark Larsen, KDWCD’s general manager.

So far, the system has mostly done its job, though there have been scattered incidents of overflow throughout the county.

“There has been flooding already,” Larsen said. “We’ve had some levee breaks and washouts in the system. It’s stressed.”

While rising waters are a cause for concern, the situation has not gotten beyond KDWCD’s ability to handle the influx so far.

“People don’t need to panic. We’re managing the water right now,” Larsen said. “There have been issues. It hasn’t been as bad as it could.”

System Could be Overwhelmed

While it’s not yet time to panic, it is time to make preparations for possible flooding as rain continues to fall through at least the end of March.

“I would be very cautious and prepare for the worst and expect the best,” Larsen said.

Some points in the flood control system simply weren’t designed to stand up to the amount of water currently flowing into the Valley. The St. John’s River in Visalia is a prime example of where trouble may lay in wait.

“There are banks on the St. Johns river along the north and south sides. They’re not engineered levees,” Larsen said. “Historically, farmers pushed dirt up on the banks and called it good.”

The berms that contain the St. John’s may not be up to the task of restraining the river once the record snowmelt reaches it.

“They’ve worked in normal years,” Larsen said. “They’re not intended for this.”

The bottom line, according to Larsen, is our waterways were never intended to handle this much water.

“We’re trying to manage flows the system wasn’t designed for,” he said.

Because of that, Larsen said people – especially those living near watercourses – should be very aware of their surroundings and water levels. He advises them to be ready to evacuate quickly and pay close attention to warnings and advice from authorities.

“Anyone living near a stream or channel should be worried,” Larsen said.

Residents Stranded by Floodwaters

So far, flooding has broken out in limited areas of Tulare County. The city of Woodlake, home to Bravo Lake, is experiencing widespread flooding for the second time this week, and near the Tule River in Porterville, homes are being evacuated as crews struggle to contain the river in its banks. Evacuations have also been ordered in and around Three Rivers and Springville.

An additional evacuation order was issued late Thursday for an area south of Tulare. The order includes the land east of Highway 99 to Road 152 (Bliss Lane), north of the Tule River to Avenue 192. Water is also collecting along the east side of Highway 99 between Goshen and Traver.

An assortment of roadways throughout the region have also been closed. That includes the Generals Highway in Sequoia National Park, which experienced a major rockfall.

According to a petition to the California National Guard and Gov. Gavin Newsom circulating online, more than 160 people are trapped on the wrong side of a washed-out bridge on South Fork Drive in Three Rivers. It is unclear who owns the bridge and who is responsible for its repair. Kacie Fleeman, a trapped resident of the affected area, is asking the state for an emergency repair.

“We are asking that the National Guard step in to do temporary repairs as soon as possible, as we have no access to any emergency services or stores for basic necessities,” the petition reads.

The petition, which claims elderly residents are stranded with no access to medications, is hosted by change.org. Repairs appear to be underway as of Wednesday.

The Return of Tulare Lake

These storm-driven flood events, however, may only be the start. As the snows melt, the immense volume of water it contains could lead to the return of Tulare Lake.

Once the largest freshwater lake west of the Mississippi River, Tulare Lake was drained during the last century as the four rivers that fed it were dammed and their waters stored upstream. At one point, Tulare Lake covered more than 1,000 square miles in and around what is today the city of Corcoran.

Most recently, Tulare Lake refilled in 1997 during a long series of powerful winter storms, however the current situation is more like that of 1983, when a record snowpack also covered the southern Sierra. That year, the return of Tulare Lake flooded more than 100,000 acres of farmland. It was two years before the land could return to production.

This year, the situation may be much more intense. The snowpack sitting in the Sierra is the largest ever recorded, and since 1983, subsidence from the pumping of groundwater during the sustained drought has lowered the elevation of the west side of the Valley. That means if flooding occurs this year, it has the potential to be wider spread than in the past, according to the Public Policy Institute of California (PPIC).

“This is likely to have two impacts. First, flooding may affect a potentially wider area that’s now lower and within reach of flood waters,” wrote Jeffrey Mount, a water policy expert and senior fellow at the PPIC. “Second, even though infrastructure that’s used to move water around in the Tulare Basin is critical to managing floodwaters, subsidence has altered the slope of many irrigation canals, reducing their capacity to move water. It’s not yet clear how this will impact flood management, but it could pose a challenge.”

The deciding factor is the weather. If temperatures rise suddenly and dramatically, widespread flooding is a significant likelihood. The USACE’s Miller said the Corps will have a better idea of how stable the snowpack is by Saturday, after an evaluation by hydraulic engineers.

“You’re going to look at how much water you have, how much saturation you have,” Miller said. “Do you have a place to put it? Do you have a way to route it safely through the system?”

As long as the weather cooperates, the USACE’s reservoirs should be able to handle the flow, he said. If the temperatures rise, the water situation will be far less favorable.

“It can come down pretty quick,” Miller said. “That’s why our goal is making room downstream.”