MODELLED IMPACT OF GROUNDWATER ROUTING ON POST-GLACIAL DRAINAGE NETWORK EVOLUTION IN THE CENTRAL LOWLANDS

A robust understanding of the landscape development of the Midwestern United States following continental glaciation is relevant to sustainable land usage in this landscape critical to food security. After the Wisconsin glaciers receded, a significant portion of the uplands of the Intensively Managed Landscapes Critical Zone Observatory (IML-CZO) was unconnected to external drainage networks and seasonal wetlands occupied closed depressions. This non-contributing area was progressively integrating into river networks like the Sangamon River of east-central Illinois prior to drainage for agriculture. We numerically model the growth of channel networks prior to intensive agriculture using Landlab, an open source landscape evolution model system. To simulate post-glacial routing of water we combine existing surface water routing tools with a component we developed to simulate Darcian flow of groundwater in the shallow subsurface. Surface and groundwater that reaches channel heads contributes to their erosion via fluvial incision. Previous work suggests that observed channel networks in Illinois and Indiana require water from upland closed depressions to contribute to fluvial incision of externally draining streams. We explore modeled groundwater routing across surface water divides as a potential mechanism for driving relatively rapid rates of channel network growth and compare the rates and morphology of modeled channel networks to observations. We supplort our proposal of groundwater travelling through surface water divides with isotopic analysis of water samples collected from an upland spring in the Sangamon River Basin over the course of a summer.