#12 - Ice Age Changes Path of the Missouri River

Map courtesy of Mountain Press Publishing of Missoula, Montana; click on map to enlarge.

Doesn't make sense . . .
The Milk River starts in Glacier Park, flows up into Canada and then returns to Montana northwest of Havre. Its valley is small, as you would expect for such a small river. However, a few miles east of Havre the valley of the Milk River suddenly becomes very wide, which is very unusual! From there to where it empties into the Missouri near Glasgow, the little river meanders along in the spacious floor of a broad valley that it could not have eroded. There is no corresponding change in resistance of the bedrock, so their had to be some explanation for the discrepancy.

A Hand-Me-Down Valley . . .
Shortly after the turn of the century geologists pointed out that the broad valley of the Milk River from Havre to Glasgow is about the size of the Missouri River valley below (east of) Ft. Peck, and that the Missouri River flows through a narrow canyon for a long distance between Ft. Benton and Ft. Peck. They suspected that the Missouri River may have occupied the broad valley along the Hi-Line until the ice pushed it south during a recent ice age. They hypothesized that after the ice age ended the Milk River found that old pre-ice age valley of the Missouri River.

Missing link found . . .
More evidence was found, supporting the hypothesis. Another oversized valley, the valley of Big Sandy Creek south of Havre, was studied and determined to be a portion of the ancient valley that connected the current Missouri to its old valley on the Hi-Line. The ancient valley was not obvious at first because it was hidden beneath gravels brought to the area by the continental glacier and then transported again by meltwater as the ice age ended. More field work revealed the final connecting segment of the abandoned Missouri valley near Havre, which was also buried beneath glacial sediments. Mystery solved!

Term: stream channel