Sakakawea reservoir on the rise

Lake Sakakawea has been on the rise for several days, so much so that water levels are on pace to exceed projections issued earlier this month. According to the U.S. Army Corps of Engineers March 1 runoff outlook, Lake Sakakawea was projected to reach 1,834.1 feet on March 31. The reservoir reached 1,833.69 feet early Wednesday and remained on the rise.

Inflow to Lake Sakakawea reached 87,000 cubic feet per second Tuesday. It was 74,000 cfs Wednesday. March 1 inflow was 18,000 cfs. Large increases in inflow are expected each spring, but the timing is difficult to predict with accuracy due primarily to variable weather conditions effecting the melting of plains and mountain snowpack.

Does the early arrival of major inflows into Lake Sakakawea mean simply an earlier rise or that there is a likelihood of greater than anticipated runoff yet to enter the reservoir? The answer may be available in the next runoff outlook scheduled to be released April 1. It is considered to be a very good snapshot of the runoff season due to fact that mountain snowpack usually peaks at mid-April.

An increased runoff outlook forecast remains possible. Runoff into Missouri River system is already expected to be well above normal according to the Corps’ March 1 outlook. That outlook calls for 30.6 million acre feet of snowmelt runoff to enter the system. The long-term average is 25.2 maf.

Although runoff is projected to be higher than normal, the amount is not considered alarming by the Corps which categorizes this year’s runoff as a “one in four” event and points out that there is ample storage in the Missouri River reservoirs to contain projected runoff. On March 1 the Corps projected that Lake Sakakawea would peak in July at 1,845 feet. Spillway level is 1,854 feet.

Missouri River watchers vividly remember 2011 when Lake Sakakawea reached the overflow point for the first time in the history. The Corps says a repeat of 2011 is “highly unlikely,” saying it was a combination of heavy mountain snowpack, heavy plains snowpack and heavy spring rains that created over 60 maf of runoff in 2011.

While this year’s plains snowpack was minimal, the amount of mountain snowpack continues to require closer examination. Surprisingly, the amount of current snowpack water content approached 2011 levels in late February and has slightly exceeded 2011 levels thus far in March. Whether or not the trend continues is dependent upon unpredictable weather.

The rising mountain snowpack was reflected in the March 1 outlook which called for 121 percent of normal runoff into the Missouri River basin this spring. Since that time though, additional snowpack has accumulated. Peak accumulations are generally achieved by April 15.

The latest measurements conclude that the snowpack “total above Fort Peck” had risen to 133 percent of the 30-year average and that the “total Fort Peck to Garrison” snowpack water content had reached 137 percent of average. For comparison, the March readings were 122 percent and 133 percent respectively.