Reconstructions of preglacial drainage in the upper Mississippi Valley region have previously been based on the pattern of bedrock valleys. It has been recognized recently that many bedrock valleys have been produced during Pleistocene time by glacial diversion and accompanying glacio-isostasy.
A more speculative approach to reconstruction of preglacial drainage in the upper Mississippi Valley region is based on considerations of paleoslopes, late Pliocene landscapes, and structural control, in addition to bedrock topography. This approach results in a pattern that is in harmony with the underlying geology, but it does not lead to a single, unique, unambiguous reconstruction. For the purposes of this study, the trunk stream is assumed to lie along the axis of the Wisconsin and Kankakee arches. Like that of the Wisconsin arch, the drainage pattern is asymmetrical, the longer tributaries entering from the west and north. The present course of the Mississippi River above the mouth of the Rock River is postulated to be an ice-marginal stream associated with an early glacial advance into Iowa from the north and west. A glacial forebulge may have played an important role in determining this course.