Computer generated image of the Central Mississippi River valley, the Illinois River valley, and parts of the Lower Missouri and Upper Mississippi River valleys.

Modified after an image of the central United States produced by Andrew D. Birrell ( https://www.research.digital.com:80/SRC/personal/Andrew_Birrell/reliefMaps/).

Image (modified) courtesy of Andrew D. Birrell ( https://www.research.digital.com:80/SRC/personal/Andrew_Birrell/reliefMaps/)