No flooding expected for mighty Missouri River
MITCHELL, S.D — After unseasonably warm temperatures in February melted much of the snow in the Dakotas, experts don't expect the Missouri River to flood in South Dakota this year.
Eileen Williamson, public affairs specialist with the United States Army Corps of Engineers' Northwestern Division based in Omaha, Nebraska, said mountain snowpack has increased recently, but much of the region's plains snow has already melted, making flooding in spring unlikely.
Still, Williamson said unpredictable weather in the Midwest can change forecasts, so USACE continues to monitor snow melt and weather patterns.
"We have to watch it because you just never know. A big rain storm, the snow pack can turn around and start climbing again, so we check it and monitor it and we're reporting the snowpack information every week," Williamson said.
As of Tuesday, the most recent data available, mountain snow totals above Fort Peck in Montana were 7 percent lower than average for this time of year, according to USACE, and there was no snowpack in South Dakota, except for small amounts in the Black Hills.
The snow has been disappearing since early February, when there was up to 1 inch of water equivalent in snow across the state and as much as 6 inches in northwest South Dakota and the Black Hills.
A Tuesday measurement showed the water level in the mainstem Missouri River system rose slightly as a "result of unseasonably warm temperatures" in the past week, bringing the total water storage to 78 percent of maximum, just inside the system's flood-control and multiple-use zone but well outside the exclusive flood-control zone.
Snowpack has been disappearing more slowly in North Dakota, where up to 6 inches of liquid content remains in the north-central part of the state, 28 percent more than usual between Fort Peck Dam and Garrison Dam, north of Bismarck, North Dakota.
But that snow is unlikely to cause flooding along the Missouri River in South Dakota. Williamson said the plains snowpack up north is more likely to cause local tributary flooding, rather than stress major reservoirs.
The last major flood of the Missouri River came because of high record snowfall in the Rocky Mountains of Montana and Wyoming. That, coupled with near-record spring rainfall in central and eastern Montana, caused all six of the Missouri River's major dams — including two in South Dakota — to release record amounts of water.
According to the National Weather Service, which forecasts flooding for tributaries in the Missouri River Basin from Missouri to Montana, only one location has more than a 40 percent chance of major flooding this year.
The James River near Grace City, North Dakota, has a 61 to 80 percent chance of major flooding. The river has between a 5 and 40 percent chance of major flooding at all locations through South Dakota, which is fairly typical.
Even in South Dakota, the James River, which runs east of Mitchell and connects with the Missouri River near Yankton, faces higher probabilities of minor flooding than other tributaries in the state. The White River southwest of Oacoma has an 80 percent chance of minor flooding, but the rest of the tributaries adjacent to the Missouri have a 20 percent chance or less.
South Dakota's snowpack could return with a snowstorm that began Thursday and is expected to end later today after dropping an estimated 10 inches of snow around Mitchell.
But because South Dakota is in the "pothole region" of the country, Williamson said much of that eventual snow melt could collect in little pools instead of flowing to tributaries, so USACE will monitor water levels as the weather warms.
While flooding is not a concern for now, Williamson said there could still be plenty of water for recreational activities on South Dakota's largest river, especially if the region is hit by strong spring rains.
"We can look at what the snowpack does and how much that shows up in the reservoirs, but as history has shown us, you can't base everything on the snowpack," Williamson said. "You have to watch the weather as well."