Extremely wet soils and prospects for at least normal precipitation have the U.S. Army Corps of Engineers continuing to raise the specter of possibly heavy spring flooding again in 2020. During a conference call webinar Feb. 6, USACE and National Weather Service specialists again called attention to the flood threat.
"Almost the entire Missouri (River) basin is above normal on soil moisture," said Kevin Low of the National Weather Service Missouri River basin forecast office. Low noted that the soil moisture content in North Dakota and South Dakota is in the 99th percentile. "This means a much higher efficiency for snow melt or rain," he said. In other words, precipitation that falls has nowhere to go except to run off. Low said this soaked-up soil means a higher risk for flooding, along with a higher base flow of the Missouri River itself and tributary rivers and streams. "The base flow is water in the channel itself," Low said. "The channel flow doesn't go up and down -- it's maintained; it doesn't recess. It's being continuously fed by the (river) banks."
The loaded-to-the-brim soil moisture profile is one factor that would lead to significant spring flooding. Other factors that would add to the threat, should they develop, would be: formation of above-normal snowpack; a long-lived, widespread cold snap on bare ground that would freeze the soil to a deep level; a sudden transition from winter to spring that would melt the snowpack rapidly and increase the risk of ice jam formation in river valleys; and a heavy rain event on top of the snow pack that would release the water in the snow rapidly.
2019 saw Upper Missouri Basin runoff total 60.9 million acre feet, just 0.1 million acre feet below the record 61.0 million acre feet in 2011. The Corps of Engineers forecasts the 2020 Upper Missouri Basin runoff to reach 36.3 million acre feet. If this forecast verifies, it would be the ninth-largest runoff year in 122 years of recordkeeping.
The Corps of Engineers is trying to do some advance preparation for possible flooding, including what Missouri Basin Water Management chief John Remus called an aggressive release of water from the Corps-managed reservoir at Gavins Point, South Dakota. "The heavier release means that the Corps can shut off flows or reduce them if needed," Remus said.
Forecasts for an Arctic-origin cold wave to spread over the Missouri River basin during the next 10 days through mid-February have the river officials nervous. "Rivers are running higher than normal," said Low. "A possible long, cold snap with higher base flows means a greater chance for ice jam formation and flooding."
Besides the cold, a persistent storm track from the Pacific Northwest tracking eastward through the upper Missouri Basin will also get much scrutiny. Mountain snowpack levels for Missouri River dams at Fort Peck, Montana and Garrison, North Dakota were very close to average as of late January.
To sum things up, Doug Kluck, NOAA Central Region forecaster from Kansas City, put a human characteristic spin on the saturated soil profile. "It's also called 'memory' in the system," Kluck said. "There's quite a bit of memory in the system (for flooding) so it's going to be high for a while."
Here are the top 10 Upper Missouri Basin runoff volume totals and the years recorded:
1) 61,004 kAF, 2011
2) 60,871 kAF, 2019
3) 49,037 kAF, 1997
4) 42,077 kAF, 2018
5) 40,634 kAF, 1978
6) 38,676 kAF, 2010
7) 37,160 kAF, 1995
8) 36,988 kAF, 1927
9) 36,203 kAF, 1986
10) 36,156 kAF, 1993
The forecast 36.3 million acre feet runoff in 2020 would knock the 1993 flood year amount out of the top 10 in the Upper Missouri basin.
Bryce Anderson can be reached at firstname.lastname@example.org
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