WHERE FLOWED THE KANSAS AND MISSOURI RIVERS DURING MAXIMUM EXPANSION OF THE LAURENTIDE ICE SHEET?

The Kansas River occupies a nearly straight, 210km-long valley that joins the Missouri River, which then continues another 130km on the same orientation through western Missouri. These reaches approximately coincide with the southerly limits of the Laurentide Ice Sheet during what has been called the Kansan and at least three older glaciations, the farthest southwesterly advances of mid-continental ice. In detail, however, the glacial ice actually crossed these streams and approached, or even surmounted, the major drainage divide 3 to 13 kilometers to the south. Ice filled the pre-existing Kansas River Valley, impounding a long lake. Low-volume drainage proceeded eastward to the Missouri River through a series of lakes and ponds connected by small cols. The record along the southern edge of the Missouri Valley is very similar: ice extending southward across the valley, locally topping the adjacent southern divide, and leaving till and lakebeds. Throughout this distance one question is prominent: where did the Kansas and Missouri Rivers flow when their valleys were filled by ice? Upstream from the Kansas-Missouri River confluence the Missouri Valley is nearly straight for almost 480km south-southeast from the northeast corner of Nebraska. As in Kansas, the western boundary of the mid-Pleistocene ice sheets was sub-parallel to the major drainage of the Missouri River. However, here the separation is on the order of 160km, yet no evidence of a truly through-flowing ice-marginal stream has been recognized. Again, where did the paleo-Missouri flow to circumvent the ice sheet? During times of maximum extent of the Laurentide Ice Sheet, precipitation continued to fall on the southerly Great Plains and was topographically constrained to flow eastward to the axial drainage of the Missouri and Mississippi Rivers and their western tributaries, but many of the details remain to be discovered.