The upper Missouri River basin may be in for more rain, more flooding and more high dam releases as fall turns to winter.
On Thursday, the U.S. Army Corps of Engineers (USACE) Northwestern Division, Missouri River Water Management Division held its monthly conference call to discuss the status of the river basin and expectations for the coming months.
Kevin Low with the National Weather Service (NWS) said the last month has been largely like the rest of the year.
“The past month since our last call has remained hydrologically active,” Low said. “We have had precipitation across the entire Missouri River basin with significant rainfall occurring over eastern Montana, all of North Dakota, eastern South Dakota, across Iowa and into northern Missouri.”
Where that precipitation has been falling, it’s been coming down in massive amounts with some areas receiving more than 400% of normal rainfall since the last Corps conference call in September. He said that some places have received from 6-10 inches of precipitation in that time frame.
Low said it’s not just rainfall, either.
“We’ve also developed an early-season snowpack over much of the upper basin including Montana, Wyoming and the western Dakotas,” he said. “Estimates of plains snow-water equivalent — that is, the water contained in that snowpack — vary from a trace upwards to an inch.”
However, he said the current storm impacting the plains may end up bringing much more precipitation the waterlogged region.
“We are now watching an evolving winter storm extending from southeastern Montana into the Dakotas,” he said. “This winter storm will significantly impact the eastern half of the Dakotas. It’s already started and will go through Saturday. The snow-water equivalent associated with this evolving system in the Dakotas could bring as much as 3 inches of snow-water equivalent.”
Low said this could mean additional troubles for areas that are already dealing with flooding.
“In the state of South Dakota, we have minor flooding along the James River and minor flooding occurring along the Big Sioux,” he said. “As temperatures moderate toward the latter half of October, the snow melt from this week’s winter storm will result in renewed flooding in the North Dakota portion of the James basin and will add to the ongoing flooding that’s already occurring in the lower James River in South Dakota.”
He said the expectation is for quieter weather for much of the week after the storm’s departure on Saturday, but long-range forecasts do the area no favors.
“Unfortunately, the long-range outlooks favor the odds of wetter-than-normal conditions remaining in the Missouri basin at least through January,” he said.
Kevin Grode, reservoir regulation team lead with the Missouri River Basin Water Management Division, said that the 2019 calendar year runoff forecast for the upper Missouri River basin above Sioux City, Iowa, is 61.0 million acre feet (MAF), or 2 1/2 times average. As of Oct. 10, the upper basin runoff observed was 54.7 MAF If realized, this would match the record set in 2011.
“Continued heavy and widespread rain in South Dakota resulted in record September runoff in the lower main-stem reaches,” he said. “The runoff from Gavins Point to Sioux City was more than 16 times the long-term average and more than twice the previous record.”
Mike Swenson with the Missouri River Basin Water Management Division said this runoff almost assures the releases at Gavins Point Dam will remain at 80,000 cubic feet per second (cfs) throughout the fall season.
“The basic study has Gavins Point releases held at 80,000 cfs through November and then stepping down to the winter release rate of 22,000 cfs by mid-December,” Swenson said.
Winter conditions along the river do not allow for higher releases.
John Remus, chief of the Missouri River Basin Water Management Division, warned that conditions may force the Corps to release even more at some point.
“We do have a lot of water that’s already come into the system and we’re expecting quite a bit more,” Remus said. “Our goal is to evacuate all of the flood-control storage in the system. We are monitoring this on a daily basis — sometimes a couple of times in a day. With the precipitation that has taken place and — as Kevin Low said — there’s an increased chance of above-average precipitation in the basin through the end of the year, we may have to go up. We are doing everything we can to prevent that, but there is always a chance we may have to increase system releases.”
He gave no indication that an increase in system releases was imminent.
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