YANKTON, S.D. (AP) — With anticipated continued drought, flooding likely won’t be an issue this year in the upper Missouri River basin, according to the U.S. Army Corps of Engineers and other federal officials.
Despite recent rainfall, the Yankton region remains a combination of abnormally dry and moderate drought conditions, according to the U.S. Drought Monitor. A similar situation is found in much of the upper basin.
Reservoir inflows in the basin above Sioux City, Iowa, were well-below average in March. The updated 2021 upper basin runoff forecast is 21.3 million acre-feet (MAF), 83% of average.
For the most part, the open winter provided much less runoff, according to Kevin Low with the Missouri River Basin Forecast Center.
“The flood risk over most of the Missouri River basin is much reduced because of the below-normal snowpack,” he said. “You have the lack of plains snow and near average for mountain snowpack.”
While some parts of the basin have received recent rainfall, the overall condition remains dry, said John Remus, chief of the Missouri River Basin Water Management Division.
“Abundant precipitation fell during March in the lower basin below Sioux City, Iowa. However, March precipitation was less than 50% of normal over much of the upper basin,” Remus said.
“Due to the lack of plains snowpack in 2021, below-average mountain snowpack, and dry upper basin conditions, we expect upper basin runoff to be below average.”
The runoff forecast is based on soil moisture conditions, plains and mountain snowpack and long-term precipitation and temperature outlooks, the Yankton Press and Dakotan reported.
Those conditions may not change much in the coming months, Low said.
The outlook for April-June calls for warmer-than-normal temperatures along the entire basin, Low said. The precipitation outlook shows odds favoring below-normal precipitation to the west but equal chances of above-, below- and normal throughout the rest of the basin.
The April-June outlook also calls for below-average precipitation with similar conditions expected beyond that time period, Low said.
“It looks like late summer — July, August and September — will be warmer than normal over the entire basin,” he said. “The precipitation outlook during the same three months suggests the entire Missouri River basin will see less than normal precipitation over the summer months.”
A La Nina weather pattern out of the Pacific Ocean is expected to fade into neutral this spring, Low said. “Whether we have a La Nina or El Nino plays a very small role in forecasting summertime weather in the basin,” he said.
Mountain snowpack in the upper Basin is accumulating at below-average rates. The April 1 mountain snowpack in the Fort Peck reach was 88%, and the mountain snowpack in the Fort Peck to Garrison reach was 94% of average.
By April 1, about 97% of the total mountain snowpack has typically accumulated. Currently, plains snowpack in the upper basin is light.
The reservoir system storage is currently 56.1 MAF, at the base of the annual flood control zone. The system is positioned to serve all congressionally-authorized purposes during 2021, including flood control, navigation and water supply.
Gavins Point Dam releases were increased near the end of March to begin flow support for Missouri River navigation. During the past month, Gavins Point saw average releases of 20,200 cfs. The current release rate stands at 29,000 cfs, while the forecast release rate is 29,500 cfs.
Remus said Gavins Point should continue to meet flow targets for navigation and continue operating as normal during repairs on lower Missouri River structures.
Fort Randall Dam at Pickstown, directly upstream of Gavins Point, saw average releases of 14,300 cfs during the past month. The figure allows for any tributary inflows between the two dams.
Fort Randall releases will be adjusted as necessary to maintain the desired reservoir elevation at Gavins Point and to back up Gavins Point releases.
Kevin Grode and Mike Swenson, both with the Missouri River Water Management Division, outlined details about meeting the needs of the system.
“The Corps’ main priority is regulating the mainstem reservoirs so they operate safely,” Grode said. “The operational decisions for the basin are driven by runoffs. The system is regulated to meet the eight congressionally-authorized purposes.”
Flood control remains a priority, along with navigation and water supply, Grode said. Besides fulfilling those needs, the Corps also serves the other five areas of hydropower, fish and wildlife, irrigation, water quality control and recreation.
“In making these operational decisions, (the Corps) will comply with all applicable federal laws,” Grode said.
The Corps vacates water from the flood control and multiple use zone during the later summer, fall and winter, Grode said. The system will continue serving the authorized purposes while preparing the reservoirs for the following year’s runoff.
The system receives 75% of the annual runoff from March to July, Grode said. The records of the annual runoff above Sioux City date back to 1898. The basin has seen great variability during those 123 years, but the 2021 forecast runoff would fall in the lowest one-fourth of all time, he added.
Swenson provided an update on the current reservoir conditions and the expected results for the authorized purposes in 2021.
As of April 5, the Fort Randall elevation was 1,354.3 feet above the base of the flood control zone and very close to the normal operating level of 1,355 feet this time of year.
All the 2020 flood water was evacuated from the reservoir system by mid-December, Swenson said. However, flooding can still occur due to downstream rainfall. The ability to reduce storages diminishes farther downstream because of the increased travel time and the uncontrolled drainage over large unregulated areas, he added.
Looking ahead, the river should be able to maintain a full navigation season, provide about 9.5 billion kilowatt-hours of hydropower, which is average, and provide recreation, water supply and irrigation, Swenson said.
In addition, the system holds enough storage to take care of fish and wildlife, including endangered species, he added.
The Corps conducts reservoir storage checks July 1 for scheduling the second half of the year and Sept. 1 for the winter flow release levels, Remus said.
However, the Corps evaluates its releases and forecasts every week, he said. If dealing with drought, the Corps looks at ways of meeting needs, he added.
“When it’s below-normal runoff, we look at what is needed to be released every day to meet those targets, and that could fluctuate on a daily basis,” he said.
“It’s a zero-sum game. There is only so much water. If we release more water in April or May than we would normally, we have less water to release in August and September.”