Missouri snowpack above average

The amount of snowfall over the Missouri River basin that feeds into Lake Sakakawea is tracking higher than the 30-year average. Snowpack in the mountains of western Montana that produce a primary influence for Missouri River flows doesn’t peak until approximately April 15 but, as of Feb. 5, the snowpack was slightly above normal.

According to the U.S. Army Corps of Engineers, the mountain snowpack from “Fort Peck to Garrison” is 10 inches. That amount is considered 111 percent of the 30-year average. The reach is primarily the runoff that will enter Lake Sakakawea via the Yellowstone River.

The Corps lists snowpack in the “Total above Fort Peck” at 11.3 inches, or 106 percent of the 30 year average. That reach is what is expected to enter the mainstem of the Missouri River above Lake Sakakawea.

For purposes of comparison, the amount of snow over both regularly monitored drainages are higher than they have been the past two years. According to the Corps, snowpack above Fort Peck was 79 percent of normal at the same time in 2012 and 101 percent above normal in 2013. From Fort Peck to Garrison, the totals were 96 percent of normal in 2012 and 91 percent of normal in 2013.

The Climate Prediction Center’s one-month precipitation outlook issued Jan. 31 calls for above to well above normal snowfall over nearly all of Montana for February. The area covers all of the Missouri mainstem drainage system, meaning the potential exists for a further increase in runoff projections.

The early projections for runoff, which are certainly subject to change in the coming weeks and months, call for 26.7 million acre feet of runoff into the Missouri River. The long-term average is 24.8 maf. Based on the assumption of 26.7 maf of runoff, early projections have Lake Sakakawea peaking this summer at 1,841.1 feet in late July, five feet higher than the reservoir reached in 2013.

With much of the snowfall season remaining, to be followed by spring rain, much can change regarding the amount of actual runoff that reaches the basin. The amount of water being released from Fort Peck Dam in Montana, which flows into Lake Sakakawea, and from Garrison Dam, which backs up Lake Sakakawea, also greatly influences water levels.

Garrison Dam is the largest impoundment on the Missouri and is responsible for slightly more than 35 percent of Missouri River flood control storage. Fort Peck accounts for 22.6 percent. Further down river in South Dakota, Lake Oahe Dam accounts for 26.5 percent, Big Bend Dam 1.1 percent, Fort Randall Dam 14.1 percent and Gavins Point Dam, located at Yankton, S.D., 0.7 percent.

Lake Sakakawea was at 1,832.2 feet Friday. It is forecast to end the month of February at 1,831.4 feet and then begin to increase until the July peak of 1,841.1 feet. Spillway level at Garrison Dam is 1,854 feet, a level that was reached for the first time in the history of the reservoir in 2011.