Most of the rivers that cross the High Plains of the American West would hardly be recognized as creeks farther to the east. While the primary rivers that rise in the mountains, including the Missouri, Yellowstone, Platte and Arkansas Rivers deserve their title, many of the smaller rivers, heading on the High Plains Province itself, are dry for much of the year, transmitting water only after episodes of torrential rain or rapid snowmelt. Among these sandy channels are the upper tributaries of the Niobrara, Republican and Smokey Hill watersheds.
Cut off from Pacific moisture by the Continental Divide and located far from the Gulf of Mexico, the High Plains only receive copious precipitation when powerful storms draw in moisture laden air from the east, events that most often occur from February through June; indeed, this geophysical province receives less than 20 inches of precipitation each year. Yet, if we study the High Plains topography, we find that these meager conduits have managed to carve ridges, hills and valleys from the otherwise level plain, suggesting that they were more substantial streams in the past. In fact, during the Pleistocene Epoch (2 million to 10 thousand years ago) the regional climate was much cooler and wetter, giving rise to rivers that, in today's climate, have withered to channels of sand, prairie grass and scattered stands of cottonwood trees.
As with other ecosystems across our globe, it is impossible to understand the current geography without an appreciation for natural history and the region's underlying geology. In the case of the American High Plains, Pleistocene rivers sculpted the Tertiary deposits and underlying Cretaceous Sea sediments into the landscape that we find today; feeble and intermittent streams now occupy the valleys that those rivers left behind.