"The runoff in central South Dakota in May was disappointing," said Larry Cieslik, chief of the Water Management office. "It was only 30 percent of normal into the Oehe reservoir and nearly nonexistent into Fort Randall."
There is currently 38.4 million acre feet (MAF) stored in the reservoirs, up 0.5 million acre feet (MAF) from the end of April. Runoff for the year is forecasted to be 19.4 MAF, down 1.6 MAF from May's forecast. The lack of rain during the snowmelt has further reduced this year's estimated runoff. Normal runoff is 25.2 MAF. System storage at the end of June is forecast to increase to 38.9 MAF.
River flows to support navigation will remain at minimum service levels. The season length is anticipated to be shortened by 34 to 51 days, depending on runoff this month. The final decision will be based on the water-in-storage check on July 1.
Releases from Gavins Point Dam will be cycled up to 25,000 cubic feet per second (cfs) every third day during the early portion of the tern and plover nesting season. Once the plover chicks become mobile, a steady release of 25,000 cfs will be followed until it no longer meets navigation targets. Releases will them be adjusted to flow-to-target for the remainder of the navigation season.
Gavins Point reservoir will remain near elevation 1206 feet above mean sea level (msl) in June. Releases averaged 19,200 cfs in May, compared to the long-term average of 29,600 cfs. They will average 24,400 cfs in June, 6,500 cfs below the average of 30,900 cfs.
Fort Randall releases averaged 17,300 cfs in May. They were adjusted as necessary to maintain Gavins Point reservoir near its desired elevation. The reservoir ended the month at 1356.2 feet. It will fall a foot in June, ending the month near elevation 1355 feet msl.
Big Bend reservoir will remain in its normal range of 1420 to 1421 feet. Releases will be adjusted to meet hydropower needs.
Oahe reservoir fell less than half a foot in May, ending the month at elevation 1577.0 feet msl, where it is expected to remain in June, about 29 feet below normal. The reservoir is 0.5 feet higher than it was last year at this time.
Garrison releases averaged 15,300 cfs during May, compared to the long-term average of 21,600 cfs. They are expected to average 21,000 cfs in June, compared to the 23,400 cfs average. This is a change from the May study, which showed a June release of 19,000 cfs. The change was made necessary by the extremely low runoff into the Oahe last month. The increased release will better balance the water in the upper three reservoirs and reduce the risk of water intake problems at Oahe this summer. Garrison reservoir rose nearly 2.2 feet in May, ending the month at 1814.7 feet msl. It will rise a foot to 1815.7 feet in June, ending 24 feet below normal. The reservoir is almost 6 feet higher than last year at this time.
Fort Peck releases averaged 6,900 cfs in May, compared to the long-term average of 9,300 cfs. They will average 8,000 cfs in June. The reservoir rose 1.5 feet in May, ending the month at elevation 2204.9 feet msl. It will continue to climb 1.4 feet in June, ending the month at elevation 2206.3, about 28 feet below normal. It is 5.3 feet higher than last year at this time.
The six main stem power plants generated 529 million kilowatt hours (kWh) of electricity in May, only 65 percent of normal because of lower pool levels and reduced releases from the dams. The forecast for energy production in 2006 is 6.1 billion kWh, compared to the average of 10 billion kWh.