There is no more impressive, and more accessible, illustration of the power of the great Ice Age glaciers than the changes in the course of the upper Missouri River.  The evidence is easy to see and understand, once we realize what has happened.

The river systems of the Saskatchewan and Missouri Rivers are very ancient.  At the time of the dinosaurs there was a shallow sea filling most of the region, and the rivers flowed directly into that sea.  About 60 – 70 million years ago the Laramide orogeny begain raising the Rocky Mountains and filling in the sea.  When the sea became filled with sediments the rivers had to flow across the plains; some ended up in the Arctic Ocean others flowed toward the Gulf of Mexico.  The location of the divide between those drainage systems is not known precisely; but it seems to have been far to the south of the present day divide.  It seems to have run eastward from the rapidly rising mountains of Wyoming, through the Black Hills toward the hills of central Minnesota. The South Saskatchewan and Missouri River systems were at one time connected.  The vast system comprising the North Saskatchewan, the South Saskatchewan, and Missouri Rivers flowed toward the Arctic (before the formation of Hudson's Bay).  The map shows a plausible reconstruction.

Click here to display enlarged map with roads and annotations. From Delorme Maps

The present-day drainage divides are shown on the enlarged map as black dotted lines.  The pre-glacial rivers flowing in their present courses are shown in light blue, and the pre-glacial rivers that have been shifted out of their course are shown in dark blue.

The relationships between the Missouri and Saskatchewan River systems have been confused by the great continental glaciers that once extended almost to the point where the Missouri now flows into the Mississippi.  There are many peculiarities in the present river courses: there are great valleys running across the prairies that seem to be unrelated to the major rivers.  Some of those valleys have only insignificant streams running in them today.  The North and South Saskatchewan Rivers both flow in a southeastward direction out of the mountains; then turn abruptly to the north.  Topographical maps show that there is a broad valley continuing toward the southeast from the bend of the South Saskatchewan.  That valley connects to the Q'Appelle Valley; but the Q'Appelle River flows in the wrong direction, away from the South Saskatchewan.  Likewise, the small, sluggish Milk River flows for several hundred miles through a valley that is much too large to have been excavated by that river.  The alignment of valleys on topographical maps suggests that the pre-glacial South Saskatchewan turned southeastward to join the Missouri, rather than northward to join the North Saskatchewan—as it does today.

The North-South Divide between the river systems has shifted toward the north (shown as a dotted line on the enlarged map).  The Divide now runs from a small mountain in Glacier Park, Montana, just north of the source of the Milk River, then through the Cypress Hills of Alberta and Saskatchewan.  There it follows a low ridge separating the Saskatchewan River system from the Frenchman River; the Frenchman River is the northernmost tributary of the Missouri-Mississippi system.  From the Cypress Hills the divide runs eastward to become the divide for the Souris-Assinniboine-Red River system.

But when the great ice ages began more than a million years ago, the rivers lay in the path of the advancing glaciers.  Wherever the glaciers encountered north-south oriented valleys, they tended to flow along those valleys and broaden them.  The glaciers also blocked many of the rivers, which were diverted around the southern margins of the glaciers.  In several places large segments of the Missouri River were displaced to cut a deep valley through rugged elevated terrain far to the south of its original channel.  The branches of the Saskatchewan River system were cut off from the Missouri, and eventually excavated new courses toward the north, over the depressed terrain that had been scooped out by the glaciers.  The old valleys can be seen clearly on a topographic map; the DeLorme Topographic Atlases of Montana and North Dakota are highly recommended.

See also a landforms map

Our field trip starts at the point where The Missouri River exits the Mountains southwest of Great Falls, Montana.  Approximate coordinates of important overlooks are noted in square brackets: [...].

Stops along the way:

1. Great Falls to Fort Benton
   The new Missouri Valley incised in the old.  The course of the River has not been greatly altered over the past million years here, but below the Falls it has excavated a deep new channel.
2. The Shonkin Sag
   A great overflow channel cut by the Missouri during the Ice Ages.  It has been suggested that the Sag was created in a brief catastrophic overflow event, but its immense size suggests that it must have been formed over a period of many thousands of years.
3. Fort Benton to Big Sandy
   Views of the young Missouri Valley incised in the old broad valley.  The River here has not only cut a deep, young valley, but has also been diverted toward the east.  The former course was along the valley in which the town of Big Sandy lies.
4. The Missouri Breaks
   A deep canyon cut by the Missouri after the Ice Ages.  The glaciers diverted the Missouri to the east; since the Ice Ages the River has excavated an impressive canyon a thousand feet deep in places.
5. Big Sandy to Havre
   The pre glacial channel of the Missouri.  Today this is a broad valley in which only several insignificant streams flow.
6. Northwest to Alberta
   The Milk River and diversion channels running toward the south.  The Milk River has had its course altered several times by glaciers of the Ice Ages.  Its present course takes the river across an anomalous ridge of high ground, rather than through the easier channel which would have connected it to the South Saskatchewan River.
7. Havre to Glasgow
   The Milk River occupying the old valley of the Missouri.  This is a classic example of an underfit stream, meandering across a valley that is much too large.
8. Eastern Montana
   The joining of the pre-glacial Missouri and Yellowstone Rivers.  The ice age glaciers were especially active in Eastern Montana and North Dakota.  Then have shifted both the Missouri and Yellowstone far toward the south, away from their previous course toward Hudson's Bay.
9. The Missouri Coteau in North Dakota
   Bluffs left by glacial surges.  Most people think of the Dakotas as being mostly flat; but there are several impressive escarpments that were created at the edges of vast glaciers.

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