Missouri Reservoirs Continue To Rise

The Corps of Engineers will maintain higher-than-average releases from Gavins Point Dam into the Missouri River over the next few months, the Corps’ Missouri River Basin Water Management Division said June 5.

“Due to the water currently being stored in the reservoirs and the higher-than-average runoff being forecast in the Missouri River Basin above Sioux City, Iowa, the service level will remain 15,000 cubic feet per second (cfs.) above full service to facilitate the evacuation of stored flood waters,” said John Remus, chief of the division. “The increased service level means that Gavins Point releases will be increased from 42,000 cfs. to 44,000 cfs., and possibly higher, as downstream tributary flows recede.”

Gavins Point releases will be adjusted, if needed, in response to basin conditions, the division said. When necessary, the Corps will reduce releases from the system projects and utilize the available flood control space in the reservoirs, in order to lessen flooding downstream of all the projects. However, the ability to significantly reduce flood risk along the lower Missouri River diminishes at locations further downstream due to the large uncontrolled drainage area and the travel time from Gavins Point Dam, the Corps said.

The 2018 runoff forecast in the Missouri River Basin above Sioux City, Iowa, is 34.6 million acre feet (maf.), 136 percent of average. May runoff in the Fort Peck and Garrison reaches was the second highest on record.

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“The updated runoff forecast reflects the rapid melting of mountain snowpack in the upper basin as well as widespread rainfall throughout the basin,” Remus said.

As of June 1, the mountain snowpack was 91 percent of average in the reach above Fort Peck and 88 percent of average in the reach from Fort Peck to Garrison. The mountain snowpack peaked on April 19 in the Fort Peck reach and on April 15 in the Fort Peck to Garrison reach. Compared to 2011, the mountain snowpack in 2018 had a lower peak snow water equivalent in both reaches and melted earlier. Both reaches have approximately 6 inches of snow water equivalent remaining while at this same time in 2011, there was 19 inches remaining above Fort Peck and 22 inches remaining in the Fort Peck to Garrison reach.

The Missouri River mainstem reservoir system storage was 63.4 maf. as of June 1, occupying 7.3 maf. of the 16.3 maf. flood control zone. “More than 50 percent of the system’s flood storage remains available to capture runoff from the remaining mountain snowmelt and summer rainfall events,” Remus said. “On June 1, 2011, only 12 percent of the total flood control storage was available. The considerable amount of vacant flood control storage provides flexibility to lessen downstream flooding should suddenly-developing large rainfall events occur anywhere in the basin.”

The Corps said that Gavins Point Dam releases averaged 39,500 cfs. during May. Releases were increased to 42,000 cfs. during the month. Downstream conditions permitting, releases will be increased until they reach the expanded navigation support levels, which are estimated at 44,000 cfs., the Corps Said.