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Projected runoff into Lake Sakakawea declines once again

Outlook reflects dry conditions

How low can it go? That question will likely be asked more and more in the coming days as the projected runoff outlook into Lake Sakakawea takes another decline. For the third straight month the U.S. Army Corps of Engineers forecast of runoff for the Missouri River Basin has been revised downward.

In their March release of projected runoff, the Corps now forecasts a runoff total of 21.7 million acre feet of water for the Missouri River Basin above Sioux City, Iowa. A month ago that projection was 22.9 maf. Two months ago 23.1 maf.

The reason why? Significant factors are less than usual snowfall and very dry conditions throughout the drainage area.

Each year Lake Sakakawea and other reservoirs in the Missouri River system are drawn down in preparation for snowmelt runoff and spring rains as a protection against possible flooding, one of the purposes of the reservoirs. As mountain snows begin to melt and enter the drainage Lake Sakakawea’s water level rises, usually peaking in late July, before beginning a slow decline for the remainder of the calendar year.

Last year’s Missouri River Basin runoff was approximately 30.6 maf, greater than the long-term average of 25.8 maf. This year’s current runoff projection of 21.7 maf means less water for Lake Sakakawea, though not out of the norm for the reservoir.

According to the Corps’ latest projections, Lake Sakakawea should peak this summer at just under 1,843 feet. Last year Sakakawea peaked about 1,844.5 feet. Wednesday’s reservoir elevation was slightly under 1,837 feet.

While a projected 2021 peak of 1,843 feet is not by itself alarming, there are some indicators that the three-month trend of declining runoff forecasts will continue. According to the Corps, recent warm temperatures have melted off most of the plains snowpack with only “trace amounts” remaining. Both plains snowpack and mountain snowpack contribute to runoff projections.

Mountain snowpack, primarily in Montana, and its water content is tracking below the long-term average. As of March 1 the total snowpack above Fort Peck reservoir in Montana was rated 94% of average, the same percentage given to the reach from Fort Peck to Lake Sakakawea. Both the Missouri River and Yellowstone River influence Lake Sakakawea water levels.

February runoff into the Missouri was scant, just 46% of normal. The March forecast is for 70% of normal runoff but is subject to chance given additional precipitation or a continuation of dry weather, the latter being of particular concern due to ongoing drought conditions. Dry soils, and little or no frost, means any snowmelt or rainfall will enter the ground quickly rather than make a significant contribution to runoff.

It should be noted that the average peak for mountain and plains snowfall is mid-April. However, it would take a considerable weather change for snowfall to reach even average levels in the coming weeks. According to the Climate Prediction Center, warmer than usual temperatures can be expected throughout the Missouri River Basin through the end of March with an expectation of no more than “average” precipitation during the period.

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