Comparatively level plain, the Black Hills being the only important mountainous uplift. Some of the main streams, particularly the Missouri, the Platte, and the Yellowstone Rivers in the northern Great Plains, and the Cimarron and Canadian in the southern Great Plains, have deeply eroded drainage channels, forming comparatively level valley floors 2 to 8 kilometers wide. Elevation of the Great Plains ranges from about 600 to 1800 meters, with a gradual slope to the east averaging about 1.8 meters per kilometer. Rainfall of the region is of the summer type, ranging from about 25 cm in its northwest to as much as 65 cm along its eastern edge. The major advantage of summer rainfall is that it comes when needed, during the growing season.
Disadvantages lie in the high evaporation during hot weather and low storage efficiency.
Showers of less than 1-2 cm are mostly lost by evaporation before they can do much good, especially where a significant portion of the rain may be intercepted by foliage. Torrential rains, which often have much runoff, are common. Evaporation is about twice as high in the southern as in the northern Great Plains, and water requirement of the growing vegetation is proportionally higher. The frost-free season ranges from about 100 days at the Canada border to 200 days in Texas. Although the growing season is short in the north, summer days are long.