LATE CENOZOIC DRAINAGE REORGANIZATION OF FORMER ST. LAWRENCE DRAINAGE LEFT A LEGACY OF ON-GOING DISEQUILIBRIA IN UPPER OHIO RIVER BASIN FLUVIAL AND COLLUVIAL SYSTEMS (Invited Presentation)

Late Cenozoic glacial impoundment and long-term diversion of 5 to 12 % of the pre-glacial St. Lawrence drainage into the modern Ohio River triggered disequilibria in Appalachian fluvial and colluvial systems far beyond Pleistocene glacial limits. This upper Ohio River basin landscape adjustment continues to the present, even though most drainage reorganization occurred as result of pre-Illinioan glaciation. Slack-water alluvium and lacustrine deposits filled low-lying pre-glacial topography in many locations; some of these deposits are paleomagnetically reversed. During one or more episodes of unknown length, several high-level bedrock drainage divides became major river valleys as new drainage became integrated, while some long-established drainage routes became partly-buried abandoned valleys. Researchers have documented or proposed many potential spillway cols in the last 150 years. A few cols experienced 120 to 150 m of incision, ultimately becoming narrow reaches of the Ohio River valley; a few other cols show evidence of modest or short-lived overflow; many others seem highly unlikely spillways. High terraces along major rivers, such as the Monongahela, Allegheny, and Ohio, show post-impoundment erosion lowered almost all upper Ohio system riverbeds tens of meters below pre-glacial drainage and completely reversed stream flow in several valley segments. Tributary profiles show downstream steepening and multiple knickzones indicative of erosion in response to main-stem incision, whereas upper watershed profiles appear still graded to high main-stem terraces. The mid-watershed positions of the largest knickzones show profile disequilibria have not worked entirely through individual watersheds. Slopes adjacent to streams are over-steepened and infamously unstable, which makes the upper Ohio River basin the most ubiquitously landslide-prone region in the coterminous United States. Most drainage history reconstructions have been based on modern landform elevations. However, isostatic depression and forebulge development likely factored in paleo-lake levels, stream profiles, and which cols became long-term drainage paths versus short-lived spillways or never occupied. If glaciation-induced isostasy and potential Quaternary Appalachian tectonics are considered, it is possible that seemingly conflicting drainage evolution models, such as those proposed for Lake Monongahela by White (1896) and Leverett (1934) are both right, but at different moments.