LONG PROFILES OF MIDWESTERN RIVERS CAN REVEAL NUANCES OF POST-GLACIAL FLUVIAL NETWORK DEVELOPMENT

The story of fluvial network development in Illinois is dominated by recent glaciation. In Illinois, the primary effect of the southern border of the Laurentide Ice Sheet was deposition: The Illinoian and Wisconsinan glaciations deposited the surfaces into which modern rivers are incised. Some river valleys, notably those of the Mississippi and Illinois Rivers, were syn-glacially formed and further developed by outburst floods from glacial lakes. The ~120kya Illinoian surface is well drained and the fluvial networks are well developed. In contrast, the significantly younger Wisconsinan surface is poorly drained and networks are often poorly developed. Immediately post-glaciation this surface would have been characterized by isolated drainage basins, largely unattached to the major rivers of the time. Today, this surface is dominated by 7 rivers, the watersheds of which encompass nearly the entire Grand Prairie at the HUC-8 watershed scale. How did the remarkably flat and demonstrably poorly drained Grand Prairie end up with so many medium-sized rivers? The primary expectation is that drainage would have formed through the bottom-up mechanism of headward incision, originating at the Illinois and Mississippi Rivers. We hypothesize an additional top-down mechanism in which fluvial networks developed first in isolation and grew connected through a combination of flood events and drainage area piracy. To investigate this hypothesis we have extracted the long profiles of the major Grand Prairie rivers. Preliminary observations suggest that signals of headward incision from large rivers and downstream-connecting flow events are both reflected in the long profiles. Some top-down events are known, like the Kankakee Torrents, which captured drainage in Northern Illinois and possibly pirated the headwaters of the Vermilion River through post-flooding incision. Similar capture events could have taken place at smaller scales, removing potential drainage area from one river by connecting it to another. Here we present the long profiles of the Grand Prairie rivers with their morphological relationships to parent material, age, and inter-moraine lakes as evidence of a more complicated fluvial development history in which contemporary but isolated drainage basins jockey for headwaters.