CONTEXTUALIZING EARLY OLIGOCENE FLUVIAL STRATA WITH HYDRODYNAMIC AND CATCHMENT PROPERTIES OF CONTEMPORARY GREAT PLAINS STREAMS

Rivers in the American High Plains are critical to communities and environments. They have high elevation catchments, grassland dominated floodplains, and an arid climate with lengthy dry seasons interspersed with short, intense, wet seasons. We deliver a comprehensive, quantitative characterization of Early Oligocene rivers (~33.9 Ma) that shaped the Great Plains during a global shift towards today’s icehouse climate, refining connections between past and present rivers. We leverage modern river and catchment information of the North Loup, Calamus, Niobrara, Platte, and Ninnescah, to integrate reconstructions of river hydro- and sediment transport-dynamics with elevation and precipitations reconstructions from the Early Oligocene (~34 Ma) White River Group deposits, Nebraska.

Hydrodynamic reconstructions from field measurements of bar geometry, grain-size and sedimentary structures yield approximate bankfull discharges of ~100 m³/s, bankfull flow depths of 2-4 m, bankfull widths of 30-45 m and paleoslopes of 5 x 10^-4. Geochemical data yield elevations 0.5-1 km and mean annual precipitation of 600 mm. While strata preserve thousands of years of river channel geometry through intermittent deposition after bankfull floods, they do not yield the granularity of modern hydrological and geospatial data. We used 24 USGS stream gages, with 14-105 year records, to estimate discharge and flow depths associated with 2, 10 and 15 year flood return periods. Using a 30 m² regional DEM, we calculated longitudinal slopes, and catchment elevation distributions. Our assembled suite of rivers include meandering, intermediate and braided channels; they possess 2-year flood depths and widths of 1-4.8 m and 7-786 m respectively, with discharges of 1-1097 m³/s. Longitudinal river slopes range from 10^-3 to 10^-4, with mean catchment elevations of 503-1498 m, and mean annual precipitation of 380-680 mm.

Hydrodynamic comparisons indicate that Oligocene rivers scale favorably with the North Loup, Calamus and Ninnescah rivers, which have mean longitudinal slopes of 10^-4-10^-3, and 2-year peak discharges of 100 m³/s associated with flow depths of 1-3 m, mean catchment elevations of 700 m, and catchment-averaged mean annual precipitation of 550 mm. Further, Oligocene deposits likely record floods with a 2-10 year return time.