High Rivers, Saturated Soil in South Dakota: A Recipe for Spring Flooding

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Southeastern South Dakota is only one heavy rain or snow storm away from major flooding this spring.

While Sioux Falls got a fraction of snow during February, there’s still an elevated risk of flooding. City, county and National Weather Service officials are preparing for almost guaranteed levels of flooding throughout the region.

“Extreme wetness” caused by heavy precipitation in recent years has set the region on the brink of overflowing, said Mike Gillispie, senior service hydrologist for NWS in Sioux Falls.

“Even though we don’t have a lot of snow left, we’re just getting into spring where we’re looking for heavier snow or rains,” Gillispie said. “We just don’t have the room to store that moisture that we would normally have had two or three years ago.”

While southeastern South Dakota has experienced some of the most ideal conditions to slow flooding threats, one major precipitation event could wash that away. Several factors have already set up Sioux Falls and surrounding areas for guaranteed minor flooding this spring, the Argus Leader reported.

Moderate flooding in Brookings, Dell Rapids and Sioux Falls are all but guaranteed, Gillispie said. The catch is that the severity of flooding will be driven by rain and snow the area receives over the next two months — and how much of it falls at one time.

“We just don’t have that storage,” Gillispie said. “Even with a little precipitation and snow melt, that’s enough water to push us up toward that moderate level of flooding.”

Due to a wet past few years, the soils around Sioux Falls are heavily saturated, the river levels are higher than normal, and the aquifers are nearly full.

Just a few inches of precipitation could lead to major flooding in the area, Gillispie said.

As of late February, Sioux Falls has a 50% chance of hitting the flood levels of March 2019, which tore through Dell Rapids, Renner and Sioux Falls. Several homes had to be evacuated in a southern Sioux Falls neighborhood near Tomar Park.

Normally, the chance of reaching that stage of flooding for this time of year would be 15%, Gillispie said.

Last year’s Big Sioux River flow rates at this time of the year were around 300 cubic feet per second. The river was at 1,200 as of Feb. 27, which is nearly 1,000 cfs more than normal.

Partnered with heavily saturated soil, the Big Sioux River is on the edge of overflowing.

The NWS issued a flood warning for the Big Sioux River at Interstate 90 between Thursday and Monday. The location’s minor flood stage river level is 12 feet, and the river level was at 11.68 feet as of 2 p.m. on Wednesday. The river is expected to rise to 13.5 feet by Monday afternoon, according to the warning.

Aquifers storing groundwater are nearly full around southeastern South Dakota. That means that the ground has little room to absorb any additional snow melt or rainfall.

Aquifers are about an extra four inches full than normal, Gillispie said.

“If soils would be normal, the first four inches of snow melt could soak in, but now there’s not that room to hold four inches of water,” Gillispie said. “That storage space isn’t available. That water will run off into the rivers.”

Minimal snow pack around southeastern South Dakota is keeping conditions stable for now, Gillispie said. Sioux Falls has had a relatively dry winter; while areas north, like Brookings, have more snow that will work its way into the Big Sioux River over the next month.

Additionally, the last two weeks have been an ideal melt, with above-freezing temperatures during the day and below-freezing overnight. That’s allowed precipitation to slowly absorb into the oil and rivers, instead of overwhelming the system in one fast melt.

Sioux Falls is expecting above-normal temperatures and minimal precipitation for the next few weeks. Seasonal outlooks are pointing to a relatively normal spring, but since March is expected to be dry, that could mean April and May might be wetter than normal.

“It’s better than above normal,” Gillispie said. “But because of how wet it is, even normal precipitation is going to cause worse than normal flooding.”

Although it’s a recipe for concern, Gillispie is less worried than he was a month ago. The chances of a repeat of March 2019’s flood are relatively low, locally. But, some minor to moderate flooding will happen.

“There are still a couple of months to get through before we’re out of the woods for the probability of major flooding again,” he said. “Until we get a chance to thaw and dry out the soil, any rain or snow has the potential to cause issues.”

City and county officials are preparing for flooding as spring makes its way into the Midwest.

“We’re expecting and preparing to have spring flooding,” said Sioux Falls Mayor Paul TenHaken at a Februeary news conference.

Sioux Falls officials have been working since fall 2019 to prepare for this year’s flooding — reviewing last year’s response, buying additional pumps, and inventorying sandbags and trap supplies, said Sioux Falls Emergency Manager Regan Smith.

Several leaders and key players in flooding response teams attended a flooding preparation workshop in late February, where they discussed emergency response protocol. Smith said that city officials remain “cautiously optimistic” as they prepare for flooding.

“We’re going to have flooding every year to some degree,” he said.