Thanks to a drier-than-predicted May, the spring runoff forecast for the Missouri River Basin was downgraded by the U.S. Army Corps of Engineers on Tuesday by about 600,000 acre feet.
“Especially in the western two-thirds of Montana, the precipitation the last 30 to 90 days was below normal — like 25 percent of normal,” said Jody Farhat, chief of the Corps’ Missouri River Basin Water Management Division. “Not getting any rain, or very little, is what caused it to be quite a little bit below normal.”
That statement was echoed by the Natural Resources Conservation Service in Montana.
“All of the basins in Montana received below to well-below average precipitation for the month of May,” said Lucas Zukiewicz, NRCS water supply specialist for Montana, in a statement. “This kept river flows mostly influenced by snowmelt runoff.”
As a result, Fort Peck Reservoir will be lucky to make it to the base of its flood control zone, the elevation at which the reservoir would normally start the spring.
As of this month, the reservoir is predicted to reach an elevation of 2,236, only 2 feet above the base of the annual flood control pool.
“I’m not sure we’ll even get that high,” Farhat said. “I wouldn’t be surprised to see it lower unless we start getting some rain across Montana.”
The Corps was under significant political pressure this year from downstream states, such as the Dakotas, after high runoff in 2011 caused flooding. With the snowpack in Montana above normal, the agency decided to evacuate reservoirs to capture the predicted high runoff, even though Fort Peck was already low following last year’s dry conditions.
Fort Peck Dam releases were increased from 8,000 cubic feet per second to 9,000 cfs during May, but won’t be dialed up to 10,000 cfs this month and through August, as had been planned, Farhat said.
“We’ll try to bring those back in to balance to be at the base of the flood control level,” she said.
The reservoir ended May at elevation 2,228.3 feet, up 2.5 feet from the previous month, but still 5.7 feet below the base of the annual flood control zone. The reservoir is forecast to rise 6 feet in June.
The mountain snowpack accumulation peaked in the reach above Fort Peck Dam at 132 percent of normal, and in the reach from Fort Peck to Garrison dams, it peaked at 140 percent of normal, according to the NRCS.
The agency called the runoff a “best-case scenario this spring” since the lack of rain prevented widespread flooding.
The NRCS is now forecasting statewide streamflows to be 128 percent of average for the June 1-July 31 time period and 154 percent of last year. The Missouri River mainstem’s streamflow is forecast at 130 percent of normal over the next month while the Yellowstone River is forecast at 135 percent of normal.
Although Canyon Ferry Reservoir was significantly lowered to accommodate spring runoff, it is now 88 percent full with inflows at 12,200 cfs and releases cut back to 4,800 cfs. All boat ramps at the reservoir are now usable.
At Bighorn Reservoir, the Horseshoe Bend boat ramp on the south end of the lake is only 1.6 feet above the minimum launch level.