PRESERVATION OF FORM: THE LONGEVITY OF GLACIAL INFLUENCE ON DRAINAGE PATTERNS

In formerly glaciated areas, modern river patterns and drainage textures are predominantly derived from glacial processes and features. This relationship is most apparent in tectonically-stable areas with thick glacial sediment, as in parts of Canada and the United States. During the Quaternary, continental glaciers repeatedly advanced over a substantial portion of the North American craton (Laurentia). To demonstrate glacial influence on drainage patterns, this study analyzed the low-relief till plains in central Iowa formed by multiple advances of the Des Moines Lobe during Middle and Late Wisconsin Episode (Marine Isotope Stage [MIS] 3 and 2). The lobe reorganized the landscape and drainage networks by altering topography via substrate erosion, deposition of sediment, and diversion of previously established rivers. Each advance left a lobate-shaped landscape where a large river flows roughly along the axis. In addition, a river is found beyond the terminal moraine where paleotopography drained towards the ice sheet. The youngest advance (~18 ka) clearly demonstrates that the modern river pattern is set up largely by moraines and their influence on meltwater. There are two broad categories of drainages on each lobe: (1) large, curvilinear rivers adjacent to moraines that enter the axial river at near right angles. (2) smaller streams on larger intermorainal zones flowing parallel to ice flow, i.e. perpendicular to moraines. Almost all drainages on the lobes are now underfit, as they occupy former meltwater channels. These patterns and relationships are also observed in nearby till plains that have not been glaciated since sometime in the Pre-Illinoian episodes (pre-MIS 8). This suggests that drainage networks formed by glacial lobes on a craton largely persist through multiple glacial/interglacial cycles if unglaciated, since only a ice sheet has the energy to override the pattern.