WHAT CAN WE LEARN ABOUT FLUVIAL EVOLUTION FROM SPATIAL VARIABILITY IN DRAINAGE DENSITY IN THE GLACIATED US CENTRAL LOWLANDS?

Before intensive agriculture, the formerly glaciated portions of the US Central Lowlands were characterized by poor drainage with significant fractions of the land surface disconnected from external drainage networks. This disorganization of drainage is attributed to continental glaciation by lobes at the southern margin of the Laurentide Ice Sheet. The time since most recent glaciation varies across the region from > 500,000 years to ~10,000 years, allowing for observation of drainage network recovery from glaciation across a temporal gradient. Distinguishing between natural channels and drainage ditches is a key challenge in making these observations because current agricultural drainage networks have drainage densities up to at least three times greater than pre-settlement networks. We use National Hydrography Dataset flowline shapefiles, filtered to identify perennial streams and include straightened reaches of natural channels, as well as General Land Office Survey maps from the early 1800s, to approximate pre-agricultural fluvial networks. We calculate drainage density on a 25 km² grid across the glaciated Central Lowlands. We use a bootstrapping technique to estimate mean drainage density, as measured on this grid, in areas defined by glacial landforms, climate zones, and proximity to the Great Lakes and syn-glacial rivers such as the Mississippi. Drainage density increases systematically with time since deglaciation within some glacial lobes. Drainage density is greater in areas close to the shores of the Great Lakes and near large rivers that drained the ice sheet margin, highlighting the importance of rivers that persist through glaciation or are formed during deglaciation in controlling post-glacial evolution. Large glacial lake beds exhibit lower drainage density than neighboring higher-relief till plains of the same age. We explore the relationships between modern climate (precipitation and temperature), pre-settlement vegetation, and drainage density while acknowledging that both climate and vegetation have varied significantly over the timescales of evolution of these drainage networks. Spatial variability in pre-agricultural drainage density provides insights into rates and mechanisms of post-glacial fluvial network evolution.