Page 2: The Evolution of a River Valley

It's story begins about 6000 years years ago as the land in the southern prairies was coming out of the most recent (not perhaps the last) ice age. As the glacier receded, melting on it's way, the waters gathered , waited, and then followed the ice. They gouged deep trenches into the gravel and shale as they fell northwards. In a particular stretch of southern Manitoba, one such stream led into the province near the southwest corner and proceeded northeast towards present day Souris. It then took a right angle turn and charged southeast, then east, before emptying into what was to become the Red River. Their combined waters then flowed northwards into the retreating sea. 

As the ice disappeared, the thick shell of the earth , feeling the effect of a much lightened load, began to rise. That threatened to block the path of the water, but it responded by digging it's trench like pathway deeper, year by year. This went on for some time ( a short time in geological terms, and eternity in human terms) and the trench widened and sank ever so slowly downwards into the earth's outer crust. A gorge was created, with steep walls - here lined with gravel, there with sand as it cut through an old delta , and further on, with shale walls as it moved through an ancient sea bed. It's appearance was not unlike those gullies one sees after a flash flood, a slash through the ground, barren and bare, jagged and fresh. But deeper and wider - much deeper and wider.

Then, as now, a river tends to grow weary of it's course and take every opportunity to strike a new one. They erode the outer banks until a new path of least resistance opens and the water follows. But in this case more powerful forces were at play. The speed of the earth's rebounding crust finally overtook the stream's effort to entrench itself and it found it's path blocked, or rather, it found an alternate route. It was "captured" by a northward bound tributary of the Assiniboine, and abandoned it's ancient spillway at a point called the Souris Bend or the Souris Elbow of Capture, just north of present day Margaret.



The Souris Elbow of Capture - looking east. The river takes a sharp left, but it used to continue straight ahead into what is now the Pembina Valley..

But it continued it's trenching, and the earth's crust kept rising until gradually both processes slowed down. The excess of meltwater which had created these super rivers was finally all drained away, leaving great gullies with only a relative trickle, wandering almost hidden in their oversized containers, to remind them of their turbulent past. And the river valleys entered the next stage of their development. 

The steep, newly-etched valley walls began to crumble. In places where the material was softer in nature - sand or gravel - they quickly (in the geological sense) lost their jagged edges and became rounded and gentle in appearance. The valley walls through much of the path from the Manitoba border to Souris are gently sloped, with the river channel wandering from side to side, occasionally cutting a steep slice out of one side or the other. But as the channel turns southeast the earth's outer layer appears to be made of sterner stuff and the river has been forced to cut more deeply into it as it descends towards the lower elevation of the Assiniboine Valley. As the water makes it's way downward at an increased rate, it rebounds from one side of the valley wall to the other and has cut sheer cliffs as it makes it's turns.