Those who drive across the Great Plains of North America (and who pay attention to the topography) note that the river valleys are generally broad and relatively shallow while the rivers, themselves, are small. Indeed, those who reside in wetter landscapes of the Midwest and East might suggest that these streams are more like creeks (small ones at that).
One thus wonders how these small rivers sculpted such broad valleys, especially in the semiarid climate of the Great Plains. The answer to this conundrum is two-fold, reflecting the natural history of the region over the past 100 million years. First, the sub-surface geology of the region is composed of "young," soft rock, from limestone deposited in a Cretaceous Sea to Tertiary sediments eroded from the Rocky Mountains. Secondly, during the Pleistocene Epoch (2.2 million to 10,000 years ago), meltwater from Continental and mountain glaciers and heavy precipitation in the peri-glacial zones fed rivers that were much larger than exist today. As the climate warmed and the flow into these streams fell dramatically, a landscape of broad valleys and small rivers was left behind.
Across the Northern Plains, where large meltwater lakes formed along the edge of the retreating ice sheets, rivers now flow through the broad, shallow lake beds and are prone to flooding during years of deep snow and heavy spring rains. Farther south, downpours from spring and summer thunderstorms may produce flash flooding along the normally dry or sluggish river beds. Landscapes across our planet are products of underlying geology, tectonic forces, regional climate and the erosive action of ice, water and wind; of course, man-made dams have significantly limited the power and incidence of floods in many areas.
