As a result of the high reservoir levels, the U.S. Army Corps of Engineers expects Missouri River releases from all of its mainstem dams will run above-average for months — possibly until November.
Gavins Point Dam near Yankton will remain high for much of the year, even if Missouri River downstream flooding continues, the Corps said Thursday. Agency officials provided updates during a basin-wide conference call with federal, state and tribal officials and the media.
Corps engineer Mike Swenson said the current above-normal releases — more than twice the average of 30,000 cubic feet per second (cfs) for this time of year — will continue for the foreseeable future.
"Gavins Point releases are at 75,000 cfs," he said. "The forecast is showing those releases through June and early July to manage the reservoir levels and to continue the evacuation of water from the reservoirs."
Fort Randall Dam at Pickstown, located immediately upstream of Gavins Point, will follow suit, Swenson said.
"Fort Randall releases are 68,000 cfs, and we expect to increase it to 69,000 or 70,000 cfs over the next few days," he said.
The long-range forecast remains in flux, Swenson said.
"Our long-range forecast was run on June 1," he said. "That (model) showed the continuation of the 75,000 cfs release from Gavins Point into July and August, then ramping down the releases to something lower like 55,000 cfs. We’ll continue to evaluate that (plan) as we move forward into June."
This spring, the Corps has gradually stepped up the Gavins Point releases in response to snowpack melt, heavy rainfall and flooding in the upper basin.
During a March 13 bomb cyclone, the Yankton region was inundated with flooding. The situation was worsened in northeast Nebraska with the Spencer Dam breach on the Niobrara River.
The Missouri River received an influx of runoff between Gavins Point Dam near Yankton and the immediately upstream Fort Randall Dam at Pickstown.
The Corps rapidly increased the Gavins Point Dam a number of times over the next day, reaching 100,000 cfs during a six-hour period. At the same time, the agency shut off releases from Fort Randall Dam. The 100,000 cfs is believed to be the second highest level of Gavins Point releases, behind the record 160,000 cfs maintained for several weeks during the 2011 Missouri River flood.
When this spring’s situation stabilized, the Corps scaled back the Gavins Point releases and resumed the Fort Randall discharge. However, the agency has maintained higher-than-normal releases to move water out of the network of mainstem dams.
The Corps had evacuated all of the 2018 flood water in anticipation of the incoming water. The full 16.3 million acre-feet (MAF) of designated flood control storage was available to manage 2019 runoff.
However, this spring’s upstream runoff continues to create the need for increased releases.
"The reservoir system storage is at 67.9 million acre-feet (MAF), which is slightly above the base of the exclusive flood control zone," Swenson said. "Twenty-eight percent of the flood control storage remains available."
At Garrison Dam in North Dakota, releases will be stepped up from 15,000 cfs to 46,000 cfs during June. The dam handles snowpack coming out of the Rocky Mountains.
HIGH WATER MARKS
At the other end of the system, Gavins Point represents the smallest and last dam on the Missouri River. Once the water leaves the mainstem dams, downstream flooding conditions can be worsened by two tributaries.
"We have flooding on the James River and the Big Sioux River in South Dakota," said Scott Dummer with the National Weather Service (NWS).
The NWS flood warning continues until further notice for the James River at Huron, Forestburg, Mitchell, Scotland and Yankton.
At Scotland, moderate flooding was occurring and was forecast to continue. At 9 a.m. Thursday, the stage was 14.63 feet compared to the flood stage of 13 feet.
The forecast calls for the James River to continue slowly falling. At stages near 14 feet, approximately 4,400 acres of farm land will be flooded between Mitchell and Yankton.
At Yankton, minor James River flooding is occurring, and the forecast calls for the river to fall below flood stage today (Friday). At 9 a.m. Thursday, the stage was 12.5 feet compared to the flood stage of 12 feet. At stages near 12 feet, significant agricultural flooding begins.
At the same time, the Missouri River remains high above Gavins Point Dam. The flood warning continues at or near Greenwood; Verdel, Nebraska; Niobrara, Nebraska; and Springfield.
At 9 a.m. Thursday, the NWS reported the following levels:
• Near Greenwood, the stage was 31.57 feet compared to the flood stage of 30 feet.
• Near Verdel, the stage was 24.86 feet, compared to the flood stage of 24 feet. At stages near 25 feet
• At Niobrara, the stage was 25.16 feet, while the flood stage is 21 feet.
•At Springfield, the stage was 11.72 feet, compared to the flood stage of 10 feet.
At the same time, the lower Missouri River basin has seen continued high flows, Dummer said. He cited flooding from Blair, Nebraska, to the mouth of the river as it dumps into the Mississippi River at St. Louis.
Jefferson City, Missouri, has nearly reached major flood stage, he added.
The situation could worsen, as lower areas of the basin could receive up to 5 inches of rain during the next three to four days, Dummer said.
If it occurs, that level of rainfall would cause smaller rivers to rise and slow down any receding of the Missouri River in that area, he added.
The current flooding has been fueled by widespread rainfall which has led to continued high runoff, the Corps said in a news release.
May runoff in the upper basin was 8.9 million acre feet (MAF), which is 267 percent of average. The average May runoff is 3.3 MAF.
May runoff was the second highest on record, only surpassed by the 9.2 MAF in 2011. Runoff in the Fort Randall Dam to Gavins Point Dam reach was 1.4 MAF, which is more than the average annual runoff for that reach.
The high May runoff increased the 2019 upper basin runoff forecast to 50 MAF. If that holds, this runoff total would be the second highest runoff in 121 years of record-keeping, only surpassed by 2011 (61.0 MAF) and exceeding the 49.0 MAF observed in 1997.
Runoff in 2018 was 42.1 MAF, which is currently third highest.
Because of the high runoff above Sioux City, the Corps doesn’t see much opportunity to cut the Gavins Point releases below 75,000 cfs anytime soon.
"We will maintain Gavins Point releases at this rate to continue evacuating water from Oahe and Fort Randall, which have used much of their respective flood storage," said John Remus, chief of the U.S. Army Corps of Engineers Missouri River Basin Water Management Division.
Oahe and Fort Randall dams are currently in their respective exclusive flood control zones. Fort Peck and Garrison are expected to enter their exclusive flood control zones during June as the remaining mountain snowpack melts.
The mountain snowpack peaked in both reaches: on April 18 in the Fort Peck reach at 105%of average and on April 17 in the Fort Peck to Garrison reach at 104% of average.
Normally the mountain snowpack peaks in mid-April. Cooler temperatures and additional snow in the mountains has slowed the melt, especially in the Fort Peck to Garrison reach.
As of June 4, an estimated 34% of this year’s peak remains in the Fort Peck reach and 56% of this year’s peak remains in the Fort Peck to Garrison reach.
"Fort Peck and Garrison reservoir levels are positioned to ensure that there is adequate flood control space to capture and manage the mountain snowmelt runoff," Remus added.
The seven-day forecast for the Yankton region calls for the possibility of lingering storms tonight (Friday) into early Saturday morning. Thunderstorms may again impact the area after 3 p.m. Saturday. An isolated severe storm is possible, especially Saturday evening.
Thunderstorms leave the forecast for Sunday, but return in an isolated manner early next week.
High temperatures are forecast for the mid-80s today and Saturday. The mercury should remain in the mid- to upper 70s through mid-week.
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