EVIDENCE OF FLOW REVERSAL IN THE LOWER ARKANSAS RIVER, USA

The Arkansas River is one of several large and important river systems stretching from the Rocky Mountains to the Mississippi River. It meanders across the Great Plains for ~1000 kms before plunging into a mountain-bound valley, confined by the Ozarks to the north and Ouachitas to the south. However, the geologic history of the Arkansas River is poorly understood, and crudely constrained to sometime between 60 ma and 100 ka. Questions concerning its path(s), potential integration of the upper and lower sections and overall history, remain. By combining knickpoint and chi (χ) mapping, cosmogenic nuclide derived erosion rates, detrital zircon geochronology, grain size distributions, tensile strength testing and tributary-mainstem angle analysis, we show that the lower portion of the Arkansas River was likely once two discrete rivers flowing in divergent directions. Our results suggest that a portion of the lower Arkansas was not integrated with rivers draining the Rocky Mountains or flowing across the high plains, but rather was a much smaller river, flowing east to west. We find upwards of 80% of all knickpoints in the region are concentrated in Ozark tributaries with anonymously large tributary junction angles, and with a median grain size eight times larger below knickpoints compared to above, suggesting adjustment to a baselevel change in a region lacking structural or lithologic variation. Additionally, preliminary zircon analysis comparing modern river sediments with late Pleistocene core data from the Mississippi Plain (Shen et al., 2012) suggests the Arkansas River was not yet integrated into the larger Mississippi system. Our analysis suggests that divide migration and stream capture caused a partial flow reversal, dictating the modern route and flow direction of the Arkansas River