It took a lot of water to raise Fort Peck Reservoir’s 134-mile length almost 4 feet in April, even though dam operators increased releases from the dam — climbing from 9,400 cubic feet per second to 11,000 cfs.
“The updated runoff forecast increased 3 (million acre feet) from April’s forecast,” said John Remus, chief of the Missouri River Basin Water Management Division for the Army Corps of Engineers, which manages Fort Peck Dam. “This was due to the continued accumulation of mountain snowpack in the upper basin as well as the delayed melt of plains snow during the first two weeks of April.
“Based on the current mountain snowpack and temperature and precipitation outlooks, runoff into the Fort Peck and Garrison reservoirs is expected to be above average from May through August,” he added.
Fort Peck’s water level now sits just 6 feet below the flood pool, an elevation of 2,240 feet. The top of the flood control pool is an elevation of 2,250 feet.
With more water coming in, the Corps of Engineers plans to boost releases from Fort Peck Dam to 14,000 cfs this month. That’s close to the maximum that the powerhouse can handle. Unfortunately, maintenance at the powerhouse means the generators can’t handle the full 14,000 cfs, so some of the water will have to be dropped through the spillway.
Even with the increased outflow, the reservoir is expected to rise almost another 2 feet in May, climbing to a predicted elevation of almost 2,242 feet.
As of Wednesday, the Musselshell River was pumping in about 2,000 cfs into Fort Peck Reservoir, compared to a long-term average of 143 cfs for the date. The Missouri River was running at 23,200 cfs at Landusky, upstream from the reservoir. Its long-term average for May 10 is 11,000 cfs.
Downstream in North Dakota, Lake Sakakawea picks up what Fort Peck releases. In addition to the 9,400 cfs being released now, the Milk River — which enters the Missouri below Fort Peck — was running at 4,420 cfs, compared to the average of 344 cfs. The Yellowstone River near Sidney — which also feeds Sakakawea – was cruising at 29,600 cfs, compared to a long-term average of 11,700.
Irrigators must be pulling a lot of water from the Yellowstone River already because upstream at Miles City and Forsyth the river gauge read 35,000 cfs.
The 2018 runoff forecast in the Missouri River Basin above Sioux City, Iowa, is 33.2 million acre feet, 131 percent of average, according to the Corps.
As of May 1, the mountain snowpack was 135 percent of average in the reach above Fort Peck and 129 percent of average in the reach from Fort Peck to Garrison. Mountain snowpack may have finally peaked at 123 percent of average in the mountains that feed the Missouri.
Even with the above average influx of snowmelt, the Missouri River mainstem reservoir system appears to be low enough to capture runoff and rainstorms without causing flooding downstream, the Corps said.
“More than 70 percent of the system’s flood storage remains available to capture runoff from the mountain snowmelt and spring and summer rainfall events,” Remus said.