July 11, 2011
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 five 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.
"We have not seen a basin-wide flood event of this proportion since the mainstem system was put into place," said Jody Farhat, Chief of the Water Management Division here. "If it weren't for the dams, the historic and unprecedented flooding we've experienced this year would have been much worse."
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.
"The flooding we are experiencing this year is due to a combination of runoff from heavy plains snowpack, near record mountain snowpack, and much above normal rainfall experienced in the upper basin during May and June," Farhat said. "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.
Big Bend releases are currently 150,000 cfs. Releases are expected to be gradually reduced to 135,000 cfs during July. Average releases for the month of June were 142,700 cfs. The reservoir is currently near its normal elevation of 1420.
Oahe releases are currently 150,000 cfs, and are expected to be gradually reduced to 135,000 cfs during July. Average releases for the month of June were 142,500 cfs. The reservoir is near elevation 1619.4 feet, 0.6 feet below the top of the spillway gates. The reservoir is expected to end the month slightly below elevation 1618.
Garrison releases are currently 140,000 cfs. Releases will be slowly reduced during the month reaching 110,000 cfs late in July. Average releases for the month of June were 136,600 cfs. The long-term average is 22,600 cfs. The reservoir is currently near elevation 1854.5 feet. It is expected to end the month at approximately elevation 1851.
Fort Peck releases are currently 45,000 cfs. Releases will be gradually reduced to 35,000 cfs in late July. Average releases for the month of June were 52,600 cfs, compared to a long-term average of 9,600 cfs. The reservoir is currently near elevation 2250 feet. It is expected to gradually drop ending July near elevation 2247.
The reservoir releases and elevations discussed above should not be assumed to be definitive. Additional heavy rain across Montana or the Dakotas could cause releases at all projects to return to the previous peak rates.
The six main stem power plants generated 1,135 billion kilowatt hours (kWh) of electricity in June, 136 percent of normal.
For general questions regarding our flood response information efforts, please call (402) 996-3877, (877) 214-9110 or e-mail us at MRJIC@usace.army.mil.