During the last half of May, the upper Missouri River basin received nearly a year`s worth of rainfall. In addition, the estimated snow melt runoff was 212 percent of normal across the upper portion of the river system. These conditions resulted in Missouri basin reservoirs across eastern Montana and the Dakotas nearing their maximum levels. Record releases are ongoing at Gavin’s Point dam located to the west of Yankton, South Dakota. Releases are currently at 160,000 cubic feet per second (cfs). At this time releases of this magnitiude are expect to continue well into August. The previous high release at Gavin’s Point was 70,000 cfs in 1997.
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Runoff into the Missouri River Basin above Sioux City, Iowa during the month of June was the highest single runoff month since the Corps began keeping detailed records in 1898. The previous record monthly runoff was 13.2 million acre feet in April of 1952. June 2011 runoff into the Missouri River Basin above Sioux City was 13.8 maf, enough water to fill the Memorial Stadium in Lincoln, Nebraska more than 9,700 times, or once every 5 minutes during the entire month.
The May 2011 runoff into the Missouri River Basin above Sioux City was 10.5 maf, the third highest single month of runoff since 1898. The May and June combined runoff totaled 24.3 maf, just short of the normal total annual runoff of 24.8 maf.
Runoff for the calendar year is projected to reach 57.7 maf, approximately 230 percent of normal. The previous record of 49 maf was reached in 1997.
"At the beginning of the runoff season, we had evacuated all of the floodwaters from last year and the reservoirs were prepared to capture this year’s runoff. The game-changer was the rain in the upper basin that came into the system during the last few weeks of May.”
During May, the eastern half of Montana received between 300 and 400 percent of normal rainfall, more than a year’s worth of rain in some locations during a two-week span of time. Portions of North Dakota, South Dakota, and Wyoming received more than 200 percent of normal rainfall. Heavy rain continued into June with significant areas of Montana and South Dakota receiving more than 200 percent of normal rainfall. Mountain snowpack, which typically peaks April 15, continued to accumulate until early May. To date, more than 90 percent of that snowpack has melted and run off into the system.
Gavins Point releases are projected to remain at 160,000 cfs through July. Average releases for the month of June were 139,000 cfs. The long-term average is 29,800 cfs. The reservoir is near its normal elevation of 1206 feet msl.
Fort Randall releases are projected to range from 155,000 cfs to 157,000 cfs during July. June releases averaged 134,600 cfs, compared to a long-term average of 26,700 cfs. The reservoir is currently at a record elevation of 1374 feet msl, but is expected to drop to near 1367 by the end of July.