OMAHA, Neb. — The Corps’ forecast for the month of September indicates the flood water evacuation plan and release drawdown strategy is proceeding as planned.
“The four large reservoirs which captured flood water this year are declining as scheduled,” said Jody Farhat, Chief of the Water Management Division here. “Nearly 10 million acre-feet of the 16 MAF of floodwater stored in the reservoir system has been evacuated to date, and the bulk of the remainder will be evacuated by early December.”
Runoff into the Missouri River basin remained above normal in August as the Corps continued to evacuate flood waters. Barring any significant rain events during the next month, river levels are expected to return to normal in portions of the Missouri River in early October.
The 2011 runoff produced back-to-back-to-back record-breaking runoff months starting in May. June was the highest runoff month on record since the Corps began keeping detailed records in 1898. May was the third highest and July marked history as the basin’s fifth highest runoff month.
Runof f above Sioux City, Iowa during the month of August totaled approximately 3.2 million acre feet, 247 percent of normal, making August 2011 the second highest August runoff in the basin’s history. August 1993 remains the highest on record with 4.1 MAF.
In June, 13.8 MAF of runoff entered the Missouri River mainstem system. The previous record monthly runoff was 13.2 MAF in April of 1952. May runoff was 10.5 MAF, the third highest single month since 1898 and July was the fifth highest, bringing in an additional 10 MAF.
“The combined runoff for May through August totaled 37.5 MAF, more than 50 percent greater than the normal total annual runoff of 24.8 MAF,” said Farhat. “In addition, the four month runoff volume was greater than the total annual runoff in all but three years since 1898: 1997, 1978 and 2010.
Reservoir releases out of Gavins Point Dam were reduced to 90,000 cfs on Aug. 30. Releases will be held steady through Sept. 15 to prevent sloughing of water logged levees, dams, and riverbanks, and to allow for preliminary inspection and assessment of infrastructure and levees before the final drawdown to normal reservoir releases commences.
“The releases will be slightly above normal going into fall, but we’re doing our best to remain on schedule because of the importance of evacuating the stored flood water before the start of next year’s runoff season,” Farhat said.
Runoff for the calendar year is projected to reach 61 MAF, nearly two and a half times the normal annual runoff. The previous record of 49 MAF was reached in 1997. Total system storage peaked a t 72.8 MAF on July 1, making it the highest peak storage number on record in the basin’s history. The previous storage record was 72.1 MAF in 1975. System storage ended August at 63.6 MAF, down 6 MAF for the month.
Gavins Point releases will be decreased by 5,000 cfs every two days beginning Sept. 16, until they reach 40,000 cfs during the first week of October. The 40,000 cfs release rate is slightly above the typical fall release rate of 35,500 cfs. Average releases for the month of August were 136,200 cfs.
Fort Randall releases are currently at 87,000 cfs. Releases are expected to decrease to 45,000 by the end of the month. Releases for the month of August averaged 133,000 cfs. The reservoir ended the month at elevation 1361.2, down 8.1 feet from the month of July. The reservoir will continue to drop in September and is forecast to end the month near 1355 feet, down 6.1 feet from where it currently sits.
Big Bend releases are currently 77,000 cfs. Releases are expected to be gradually reduced to 35,000 cfs by the end of September. Average releases for the month of August were 113,100 cfs. The reservoir will remain near its normal elevation of 1420 during the month.
Oahe releases are currently 75,000 cfs, and are expected to be gradually reduced to 35,000 cfs by the end of September. Average releases for the month of August were 117,100 cfs. The reservoir ended the month near elevation 1613.3, 4.2 feet below the previous month’s elevation. The reservoir is expected to end the month near elevation 1609.1, 4.2 feet lower than where it currently sits.
Garrison releases are currently 55,000 cfs. Releases will be gradually reduced during the month reaching 26,000 cfs by the end of September. Average releases for the month of August were 91,100 cfs. The reservoir ended August at elevation 1844.5 feet, 7.1 feet lower than the previous month. The reservoir is forecast to end the month at elevation 1843.3, down 1.1 feet from where it currently sits.
Fort Peck releases are currently 25,000 cfs. Releases will remain constant through late September when they will then be gradually reduced to 10,000 cfs by the end of the month. Average releases for the month of August were 26,600. The reservoir ended the month at elevation 2241.9 feet, down 4.7 feet from the previous month. It is expected to gradually drop to 2238, ending September 3.9 feet lower than where it currently sits.
The rese rvoir releases and elevations discussed above should not be assumed to be definitive. Additional heavy rain in the basin could cause adjustments to the drawdown schedule.
The six mainstem power plants generated 1.187 million kilowatt hours (kWh) of electricity in August, 113 percent of normal. This year’s forecast for power production is projected to reach 11.8 billion kilowatt hours. The long-term average is approximately 10 billion kilowatt hours.
Information from the Army Corps of Engineers