Note: This story has been updated with information from the Watertown city manager.
SIOUX FALLS, S.D. (KELO) — As of March 23, there was a 95% chance that Big Sioux River on Broadway Street in Watertown would reach the moderate flood stage this spring, according to the National Weather Service (NWS).
The flood stage is at 10.5 feet at Broadway in Watertown, according to the NWS.
The water level could reach 11 feet at that point in the city, said Steve Fleegel, a meteorologist with the NWS in Aberdeen. “When it gets to 11 feet you will see (water) to the banks on both sides. Riverside Park starts to flood as things get higher,” Fleegel said.
City manager Amanda Mack said officials from the city and Codington County met this morning to discuss the NWS flood forecast.
“We’re not overly concerned but we are getting things prepared,” Mack said. The possible flood areas do have several homes and some public infrastructure that could be impacted, Mack said.
If response is needed, “We want to hit the ground running,” Mack said.
NWS information said the Big Sioux River could crest around April 12, she said.
Officials will be monitoring river gauges and the NWS, Mack said. It will also be monitoring the levels of the two lakes, she said.
Mack wasn’t in Watertown for the 2019 flooding, but “everyone else in the room (today) was there,” she said. Flooding impacted parts of the city as well as other cities along the Big Sioux River that year.
Although the pace of melting snow and any additional rain or snow could impact the flood forecast, so far the risk for major flooding is low.
“The probability for major flooding is pretty low at 5% at Watertown on Broadway,” Fleegel said.
Major flooding can cause significant damage, he said.
The NWS flood forecast on March 23 said there was a 75% chance the water level will get to 12 feet and only a 10% chance it will get to 13.1 feet at Broadway for major flooding. There is only a 5% chance it will exceed 13.8 feet.
The latest forecast only includes Watertown as the largest city in the state on the Big Sioux River that could be impacted by flooding.
Minor to major flooding is expected along the James River. The James River frequently floods in part because it travels across low-lying ground and there is only a small decline in elevation along most of its routes.
Fleegel said that in general, north and south of Aberdeen to Mitchell, “There is a greater than 50% of moderate flooding.”
In 2019, the river had the potential to be in flood stage throughout the spring, summer, and fall, into the next winter, Fleegel said.
“Even last spring in April it reached flood stage and it stayed that way all the way into September,” Fleegel said.
The flood forecast uses data from the past 30 years, current river conditions, soil moisture levels, snow cover and the 30 to 90 day outlooks for temperature and precipitation.
Right now, areas of far north central and northeast South Dakota have the most snow cover.
“We have some pretty decent snowpack in the north…,” Fleegel said. More than six inches can be found in some areas of the north-central and northeast.
Low levels of frost depth can help offset some of the snow cover.
Frost depth is the depth or level at which the soil under the surface is frozen. The soil can absorb the moisture from melting snow if the frost has thawed. The lower the frost depth, the faster the soil can thaw and absorb snow melt.
“Certainly having the soils thawed out or nearly thawed out is ideal for the spring snowmelt season,” state climatologist Laura Edwards said on March 16.
As of March 27, the South Dakota State University Mesonet weather data said the frost depth was a low as four inches in some parts of northeastern South Dakota. But snow cover was much higher in some areas. Other areas in the north-central has frost depths of 25 inches, according to the Mesonet.
Any possibility of flooding will be affected by how soon the snow melts.
Days above 32 degrees allow snow to melt while nights below freezing stop the melt for what could be described as a more manageable melt cycle.
The colder weather pattern is a positive when it comes to melting snow and a break when it freezes, Fleegel said.
“We are only a few days away from April. Typically we have some highs in the 50s in late March and in the first part of April,” Fleegel said. “At some point, we will get out of this (cold) pattern.”
Until then, the cooler weather will help with potential flooding.