Take a standard 4 feet by 8 feet sheet of 1/4-inch plywood. Lay it on a perfectly flat surface. Put three sheets of typing paper under one end. At intervals along the sides, slip six such sheets under each edgejust enough to slightly bow the plywood. Now dump a bucket of water on the high end (the one with three sheets of paper under it). Congratulations, you have just constructed a realistic hydrologic scale model of the Red River Valley that separates Minnesota and North Dakota. Now perhaps you understand why flooding forms such a large part of the history of this region.
Why does a deep snow pack in the Big Sioux River Basin not cause the same problems as the same amount in the adjacent Minnesota River Valley? Why is the growing season for Brookings, S.D., and Pipestone, Minn., a few days shorter than Willmar, Minn., which lies farther to the north? Why have Northern States Power and other enterprises concentrated their wind generating investments in Lincoln and Pipestone counties in southwest Minnesota? The answer is that the topography of the Ninth District can vary dramatically within a few milesand not just in the Rocky Mountains of western Montana.
Take the flood-prone Red River of the North. As many people know, it is a north-flowing river that starts its trek from its southern headwaters while its northern reaches are still jammed with ice. But that is only part of the story. There are other rivers that flow north that do not have the pervasive overflow problems that plague the Red.
Water flows downhill, and the valley of the Red just does not form much of a hill. Formed as the lake bed of prehistoric Lake Agassiz, the Red River Valley slopes so little as to be almost imperceptible. Starting from Lake Traverse, elevation 976 feet above sea level, it flows about 340 miles north to Lake Winnipeg, elevation 712 feet, dropping a bit over 9 inches per mile on average. But in the crucial 160 miles from Fargo to the Canadian border, the drop is only 5 inches per mile, about the same as three sheets of paper under the end of a sheet of plywood. Furthermore, once the river rises above its shallow bed, water spreads broadly across the valley floor, where in many areas the ground rises only a foot or so per mile as one moves away from the river.
The Minnesota River, flowing diagonally southeast across central Minnesota to Mankato, then veering northeast to join the Mississippi below the bluffs of old Fort Snelling, rises only a few miles from the source of the Red River. The Minnesota drops about 240 feet in 180 miles, or about 16 inches per mile, three times the slope of the Red north of Fargo. But even more importantly, throughout most of its length, the Minnesota flows through a relatively narrow, well-defined valley. Whereas there are many places along the Red where one has to drive as much as 20 miles from the river to rise 50 feet above the riverbed, in the Minnesota River Valley, one rises the same amount in one or two miles.
The Big Sioux starts just 16 miles to the southwest of the Continental Divide separating the Red and Minnesota River valleys. From this point in northeastern South Dakota, it flows 220 miles nearly due south past Watertown, through Sioux Falls, and joins the Missouri River just above Sioux City, Iowa. But in these 220 miles, the Big Sioux falls over 800 feet, or 44 inches per mile. This does not put it in the category of an alpine torrent, but is enough to make the water move several times faster than the Red; thus, a given number of cubic feet per second moving down the river takes up much less room in the Big Sioux Valley.
How can the Big Sioux drop so much more than the Red, if they start only 25 miles apart? The answer is that as one drives west from Brown's Valley, Minn., located squarely on the Continental Divide between the Red and Minnesota River valleys, the ground immediately begins to rise, climbing more than a 1,000 feet in just 10 miles. This rise is the eastern end of the Coteau des Prairies, an upland that rises in eastern South Dakota and juts into the extreme southwestern counties of Minnesota.
This upland, called the Buffalo Ridge in Minnesota, is the reason that Brookings and Pipestone have a shorter growing season than Willmar, nearly 90 miles to the northeast. Average temperatures drop about 3 degrees Fahrenheit for each 1,000 foot rise in altitude, and this temperature differential brings later spring and earlier fall frosts to farms on the Coteau, making their average growing season nearly a week shorter than areas lower down. The high ridges of the Coteau are also extremely windy, hence the proliferation of wind turbines on the Buffalo Ridge near Ruthton and Holland, Minn.