Paleohydrology of North American Catchments and Rivers at the Onset of the Eocene-Oligocene Climate Transition: Reconstructions from Fluvial Strata of the White River Group, Toadstool Geologic Park, Nebraska

Member of the Brule Formation of the White River Group at Toadstool Geologic Park in Nebraska. Our approach combined descriptive and quantitative methods to reconstruct river flow, sediment transport, channel dynamics, floodplain behavior, and catchment-scale moisture variability and ecosystem function. We found that rivers were ephemeral, with peak flow depths and widths of approximately 2.5 m and 65 m respectively. Median peak discharges were approximately 168 cms, and base flows near zero, as indicated by subaerial exposure surfaces on river beds. Floodplains were dynamic and built by frequent floods able to suspend and deposit sand up to 200 microns, with relatively short intervening periods of stasis for soil development. Environmental information recorded in n-alkane δD, δ13C, average chain lengths (ACL) was similar and primarily inherited from transported plant material. The paleo-catchment relief, estimated from variability in δD values and modern altitude-driven lapse rates, was approximately 800 meters. River channels had a gradient of approximately 3 x 10-4, an order of magnitude less steep than modern rivers in the area. This difference is likely due to the eastward tilting of the Great Plains associated with dynamic topography that initiated during the Miocene. Modern river discharges are an order of magnitude lower and current mean annual precipitation is 100 - 220 mm less than during early Oligocene time; together, these estimates indicate greater moisture availability on early Oligocene landscapes relative to today, possibly due to lower paleo-landscape elevations at the time. Our study provides a detail-rich characterization of Early Oligocene landscapes in Nebraska, offering insights into the hydrology, morphology, paleo-elevation, and relief of rivers and catchments during this period. The coordinated approach we used integrates hydroclimatic reconstructions, river and floodplain dynamics, and sediment and water fluxes, thereby bridging the timescale gap between geological records and modern hydrological data and ensuring consistency in reconstructions across subdisciplines. Our approach can support improved predictive modeling of paired climate-river dynamics through time.