Deep, wet snowpack poised to trigger flooding
Below normal temperatures have increased flooding chances along the James and Big Sioux river basins and Watertown and subsequently Lake Kampeska appear to be particularly vulnerable.
The National Weather Service in its Spring Flood Outlook released Thursday afternoon said flood chances have increased in the past two weeks.
The weather service said there is a 99 percent chance the Big Sioux River will reach moderate flood stage with a 12-foot crest at the Broadway gauge in Watertown, which is just south of U.S. Highway 212.
There is a 50 percent chance it will crest at 12.5 feet, which would make it the third highest crest since the 1997 flood which peaked at 13.67 feet.
The gauge on the Big Sioux at Sioux Conifer Road north of Watertown and just downstream of Lake Kampeska also is expected to hit moderate flood stage with a 62 percent chance of reaching at least 10 feet with the major flood level starting at 12 feet.
That means Lake Kampeska will take significant water inflows as hydrological data show the lake absorbs about 90 percent of the water coming from the Upper Big Sioux basin until the lake and river reach similar levels, at which point flows split.
Flooding on Lake Kampeska begins around 35 inches over full, and the lake is now a half-inch above full compared to about 13 inches below full at the start of the 1997 flood.
At least moderate flooding also can be anticipated downstream on the Big Sioux near Castlewood and down past Brookings.
The James River above and below Aberdeen also is predicted to hit the moderate to major level.
River flows near Columbia and Stratford are expected to be at the moderate level while it approaches the major flooding level at Ashton, the weather service said.
At least moderate level flooding on the James is expected at Huron and Mitchell as well.
Brown County Emergency Management Director Scott Meints said moderate flooding near Columbia northwest of Aberdeen primarily affects agricultural land. It would require a future rain event to cause flooding in the Aberdeen area.
Watertown has significant reason to be concerned with five to eight inches of water in the more than 20-inch-deep snowpack covering much of the Upper Big Sioux River basin. The 1997 flood was triggered by a two-inch rain falling on a snowpack containing about seven inches of water content.
These water content levels are widespread, the weather service said. “Locally higher amounts above six inches exists across far north central South Dakota, northeast South Dakota and west central Minnesota,” its report said.
They extend into North Dakota as well and major flooding is predicted in Fargo, the weather service said.
Codington County Emergency Management Director Andrew Delgado said local officials would meet soon to plan strategy.
Watertown Public Works Director Heath VonEye said flood work has started and the city must soon decide to spend about $115,000 to rent high-capacity pumps in case it needs to isolate various utility structures and lift stations.
As of late Thursday, the U.S. Army Corps of Engineers had not mobilized emergency diking and sandbagging efforts, VonEye said. Installing Hesco baskets for emergency river diking is a major undertaking, he said.
Delgado said his agency has a large supply of sandbags should they become necessary.
Factors working in the state’s favor are below normal soil moisture and generally shallow frost depths.
Additionally, Mother Nature is allowing some time for planning. The 8–14-day temperature outlook is below normal, while the precipitation outlook is slightly above normal.
If the thaw occurs gradually, the dry ground can absorb water, the weather service said.
However, the later the cold temperatures remain, the more chance of a sudden warm-up.
“The flood threat through this spring, both in location and severity, will still depend on rate of snow melt, and any future rain or snowfall,” the weather service said.
“These next three weeks will be very telling,” VonEye said.
This article originally appeared on Watertown Public Opinion: Deep, wet snowpack poised to trigger flooding
