POST-GLACIAL FLUVIAL NETWORK EXPANSION IN THE CENTRAL LOWLANDS (Invited Presentation)

Repeated advances and retreats of ice lobes at the southern margin of the Laurentide Ice Sheet disrupted fluvial drainage networks in several way including filling Pliocene valleys with sediment, shifting continental drainage divides, catastrophic carving of meltwater channels, and reversal of flow. We consider the impact of glaciation and the post-glacial response of the headwaters of fluvial networks in the Central Lowlands of North America. Prior to extensive modification of the drainage network to allow intensive agriculture a significant fraction of the upland surface was not connected to external drainage networks via downslope surface pathways. This non-contributing area (NCA) could only supply water to fluvial channels if closed depressions filled to their spill points or if groundwater infiltrated from the NCA flowed across subtle surface water divides. Rates of channel network growth implied by decreasing fractions of NCA on surfaces with longer times since glaciation suggest that water contributions from NCA are likely to have contributed to channel network expansion because headward expansion of channels into the low relief uplands would have been prohibitively slow without discharge supplied from the uplands. Numerical models provide insights into the rates and planform morphology of channel networks expanding under different scenarios for the fate of water falling on upland closed depressions. Specifically, we compare scenarios with filling and spilling of closed depressions to cases in which groundwater is directed toward growing channel heads. Preliminary results suggest that NCA in different regions may have behaved differently depending on the glacial landforms present. In addition to addressing the specifics of post-glacial channel network growth, our work also has broader implications for numerical landscape evolution modeling as it highlights the importance of the choice of treatment of closed depressions and introduces the possibility of a weaker connection between surface topography and flow routing via groundwater flow across surface water divides.