FIGURE 2.1 Missouri River basin landforms.

SOURCE: Erwin Raisz, 1954.

The basin’s northern landscapes include level to gently rolling plains and hills composed largely of glacial till. The region between the Missouri River (on the north) and the South Dakota-Nebraska border (on the south) is arid and has eroded to form deep valleys. From there to the basin’s southern boundary lie the Great Plains, with their characteristic widely-spaced streams and broad, flat valleys. In the basin’s eastern third, the plains give way to upland plateaus and gently rolling till plains. Annual rainfall varies from 8 inches in the foothills of the Rockies to over 40 inches in parts of Missouri and Iowa. Much of the basin is characterized by the cold winters and hot summers associated with a continental climate (drier in the basin’s western portions, more humid in the east). Throughout the basin, most rain falls during the spring and summer. The basin’s western rivers, such as the Marias and the Yellowstone, gain a large portion of their flow from spring snowmelt.

In eastern portions of the Missouri basin, the climate is humid continental and the vegetation is medium-height bluestem grasses, with mixed oak and hickory forest. Moving westward into more arid regions, grasses generally become shorter and more sparse. In this portion of the basin that