Lake Agassiz: Why the Valley Is So Flat

The flatness of the Red River Valley is so extreme it feels architectural rather than natural. Standing almost anywhere between Fargo and Winnipeg, you can see the horizon in every direction with almost no obstruction. That flatness is the floor of glacial Lake Agassiz, a freshwater sea larger than all of the modern Great Lakes combined that covered this region from roughly thirteen thousand to eight thousand years ago as the last ice age ended.

The lake was dammed to the north by the retreating Laurentide Ice Sheet and drained in stages as the ice pulled back. What it left behind was a lakebed of extraordinarily deep, dark, stone-free clay, some of the richest agricultural soil on the continent. The Red River falls only about five inches per mile, one of the flattest gradients anywhere, which is why the Red River floods so catastrophically when snowmelt arrives in spring.

There is simply nowhere for the water to go.