Snow pack, high lake levels raise flooding concerns

Tuesday, March 12, 2019

Officials with the National Weather Service in Sioux Falls are warning area residents of the potential for at least minor to moderate flooding due to the existing snow pack.

"For that low end flooding, the risk is very real," Sioux Falls National Weather Service hydrologist Mike Gillispie said. "With the snow pack we have out and how wet it was last year, the threat of at least minor flooding is very high going into the spring. The soils are very wet and frozen now, so any snow melt we get will not be soaking into the soils."

The National Weather Service has issued a flood watch for the Little Sioux River because rainfall amounts of 1 to 2 inches are expected across the region from Wednesday through Thursday. Snow pack in the area contains another 1 to 3 inches of water, with temperatures expected to remain above freezing.

Gillispie said minor flooding occurs in the area approximately every other year and is most visible locally with standing water in fields or farmlands. Moderate flooding extends over onto secondary roads and into townships. Major flooding, which is not anticipated at this time, can impact businesses, homes, state and U.S. highways.

"To have the rivers, particularly the Little Sioux River, flood in the spring isn't that unusual," Dickinson County Emergency Management Director Mike Ehret said. "When it does, it's generally limited to pasture land and perhaps a few roads if it gets high enough. I think the lakes will become an issue if we see the heavy rains like we did last year."

The Dickinson County Emergency Management Commission called an emergency meeting on July 2, 2018, and voted to slow watercraft to 5 mph just before the Independence Day holiday. The U.S. Geological Survey gauging stations put last summer's water levels on the Iowa Great Lakes at their highest point since 1993. Although 2018 lake levels were far from 1993's catastrophic levels, the local agencies said shoreline erosion and property damage concerned commission members enough to quickly review regulations.

The commission has already made some changes in 2019 because of last summer's conditions: Commissioners eliminated the 600-foot language for a "5 mph zone." If emergency conditions warrant, watercraft will now be slowed to 5 mph regardless of the watercraft's distance to shorelines.

For East Lake and West Lake Okoboji, the 5 mph speed limit goes into effect when lake levels reach 26 inches — or 6.2 feet on the Lakeside Lab gauge. The previous standard for restrictions was 30 inches. For Big Spirit Lake, the 5 mph limit is triggered at 22 inches (or 16 feet on the Marble Beach gauge). It was previously 24 inches.

"Just be as prepared as possible," Ehret said. "We don't know how the snow-melt will go and if flooding will ensue … or if heavy rains will be a problem again this spring and summer like last year. Plan for the worst and hope for the best."

Gillispie said spring flooding probabilities take into account the current conditions of river basins such as the snow pack, soil moisture and frost depth and form a range of possible forecasts using 50 years of data.

"It's all dependent on how fast the snow melts," Ehret said. "If we can get days above freezing and nights below freezing and minimal precipitation, I think we'll be OK. If we get a big warm-up and/or heavy rain, then that will greatly increase the flood threat."

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