America by Air: The Meandering Missouri

Editor’s Note: This article previously appeared in a different format as part of The Atlantic’s Notes section, retired in 2021.
Adam Feiges

A reader, inspired by the Orbital View last week of a braided river carrying the runoff of a small New Zealand glacier, merges two regular features in Notes:

Here is a view of the Missouri River, just north of Kansas City, that illustrates the runoff from a continental-sized glacier. The Missouri is “underfit,” meaning that the modern stream flows through a valley that was created by an ancestral river many times its present size.

The modern-day Missouri River, which has been channelized by a massive civil engineering project aimed at promoting navigation and flood control, is dwarfed by its valley, which cuts a massive scar across the middle of the country that in places is over 25 miles wide. Here it averages about 10 miles across.

As the last glacier receded, its melt-water would have filled the valley from rim to rim during the summer months. During the winter, its flow slowed down and a braided stream like the one in New Zealand left the valley mostly sand. Tremendous spring winds, created in part by the temperature variation over the diminished glacier to the northeast, created epic sandstorms that deposited several hundred feet of fine grained silt on the eastern border of the river valley. Ten thousand years of erosion have created the Loess Hills, a relatively unique geologic feature of sharp sided mounds without a rock anywhere.