THE LATE WISCONSINAN HISTORY OF THE UPPER MISSISSIPPI RIVER AND ITS TRIBUTARIES BETWEEN BRAINERD AND ST. CLOUD, MN

The average gradient of the modern Mississippi river between Brainerd and St. Cloud, Minnesota is 2.6 feet per mile, the gradient on the associated depositional terrace is 2.5 feet per mile and the gradient of the ancient channel, now occupied by the depositional terrace, is 2.5 feet per mile. The gradients of the tributary streams joining the Mississippi river in the study area ranges from 7.6 feet per mile to 2.9 feet per mile. Four major glacial lobes were responsible for the alteration of the ancient Mississippi River channel-way and filling that channel-way with outwash derived from the Wadena, Superior, Rainy and Des Moines lobes. Many sublobes were involved as well including the Pierz and Brainerd sublobes. Precambrian structural elements appear to play a significant role in the location of some of the tributary streams and on the gradient of the terrace surface. The depositional terrace following the modern Mississippi river is on average three miles wide, has sediments up to eighty feet in thickness and extends down to the Precambrian bedrock surface in several localities and may have occupied a location to the east of the current channel just north of St. Cloud. The modern channel is entrenched an average of 42.5 feet below the depositional river terrace. An analysis of the gradient of the Crow Wing River suggests that it was the primary source of melt-water during the development of the modern Mississippi channel. The Nokasippi River, a tributary stream to the east of the Mississippi, may represent a tunnel valley that was formed beneath the ice sheet, later occupied by a prominent melt-water drainage-way from the Mille Lac moraine, which follows two prominently mapped Precambrian thrust faults along a significant part of its pathway.