The Des Moines River is a tributary of the Mississippi River, approximately long to its farther headwaters, in the upper Midwestern United States. The largest river flowing across the state of Iowa, it rises in southern Minnesota, and flows across Iowa from northwest to southeast, passing from the glaciated plains into the unglaciated hills near the city of Des Moines, which takes its name from the river. It forms a short portion of Iowa's border with Missouri in Lee County. The Avenue of the Saints passes over this section.