The Blaine County Journal News-Opinion - We've Got The County Covered

Why the Missouri River used to run north of the Bear Paws

 

July 8, 2020

This graphic shows the locations of the southern edges of the glacial sheets and glacial lakes that covered much of north central Montana 100,000+ years ago. The map also shows the present-day route of the Milk (dots) and Missouri (dashes) Rivers. The pre-glacial route of the Missouri River was around the north edge of the Bear Paws and through Blaine County along a channel now occupied by the Milk River.

Several weeks ago my wife and I were returning from a trip to the Fort Peck area. Drivers who've made that trip know there are not many places to take a break between Glasgow and Malta. We remembered a state rest area located about halfway between Hinsdale and Glasgow and decided to make a quick pit stop there on our way back home to Chinook.

While waiting for my wife I began reading the informational sign at the rest stop that gave some information about the area. I was surprised to read that the area was covered by glaciers during several different periods through the centuries (per glaciologists the ice sheets made their way on to the Montana plains about 190,000 years ago and lasted for about 60,000 years). The result from the multiple glaciers was an area now strewn with erratica (random rocks carried and deposited by glaciers from as far away as the shores of Lake Winnipeg in Canada) and remaining natural ponds and water impoundments gouged out by the glaciers. Many waterways were also altered.

After the glaciers in north central Montana

Scientists believe before these major glacial events in our area the Missouri River flowed northeast from Virgelle, across the Big Sandy, and around the north side of the Bear Paws and on to Havre. From there the Missouri followed the present-day course of the Milk River to the area east of Glasgow where the Milk River now empties into the Missouri. Near Poplar the pre-glacial Missouri was diverted from its current path and eventually emptied into the Hudson Bay rather than the Mississippi River near present day St. Louis. During one of several glacier movements in to our area, the Missouri River's pre-glacial route was damned by the ice and its course changed.

With its normal route blocked, the Missouri River moved south (below the Bear Paws, for example) and followed an ice-free route. As the glaciers melted the water pooled west of our area, creating a number of huge glacial lakes that formed in the region. These lakes, (shown on the attached map as Glacial Lakes Cut Bank, Choteau and Great Falls), would rise several thousand feet above sea level. At some point the pressure from the impounded water burst through the embankments releasing thousands of acres of water. The released torrents cut new channels that existing creeks and rivers then followed as the flood waters receded.

After the glacial sheet melted, the Missouri River never went back to its original channel and continued to follow the new route to present day. As more melting occurred the Milk River took over a section of the old channel, vacated by the Missouri River, that ran from Havre to Wolf Point. Glaciologists believe that last sheet of ice retreated 11,000 to 8,000 years ago. The Milk River took over that part of the channel once used by the Missouri River north of the Bear Paws and on east to the Glasgow area.

I often wondered why the Milk River in Blaine County was situated in such a wide valley through Blaine County. Living in the Sweet Grass Hills for a time, I noticed the Milk River in Alberta generally has a more narrow valley. Learning that the Milk River follows a valley originally created by the much larger Missouri River helped explain why the valley in Blaine County is so wide. And the switch in channels also may explain why the larger Missouri River, along the south edge of the county, has a narrow valley in most places as that is a newer route carved around the glaciers.

One other result of the glaciers

Glacial Lake Great Falls was believed to have reached a height of 3,900 feet above sea level-literally a mountain of impounded water. Scientists believe that about 15,000 years ago a glacial lake outburst carved what has become known as the Shonkin Sag. The Sag is a famous prehistoric meltwater channel created from the outburst of water from Glacial Lake Great Falls. Shallow glacial lakes can still be seen along the channel that is now dry.

The Shonkin Sag is a 100 mile long dry-bed channel created when huge amounts of water burst from the Glacial Lake Great Falls, east of the city of Great Falls. It is a famous prehistoric meltwater channel starting near Highwood and running to the Judith River to the east.

The Sag begins south of Highwood (20+ miles east of Great Falls) and runs easterly in a northeasterly loop for about 100 miles to the Judith River. It varies in width from a quarter mile to two miles and is 500 feet deep in many places (see photo). The drainage takes its name from the now defunct pioneer town of Shonkin (pictured in the photo). I've never visited the Shonkin Sag, but a visit to the "The Sag" is now on my "list of must see places in the Treasure State."

 
 

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