PALEOFLOOD ANALYSIS OF AN ALLUVIAL CHANNEL IN THE SOUTH-CENTRAL GREAT-PLAINS - BLACK BEAR CREEK, OKLAHOMA

Late Holocene floodplain reconstruction of the Black Bear Creek basin in north-central Oklahoma was chronologically developed based upon slackwater deposition and radiocarbon dating of associated paleosols. Major paleoflood events occurred 3590 +/- 80 yr BP, and 1150 +/- 100 yr BP. Assumptions developed for application in semi-arid, bedrock channels were tested and modified. Maximum accumulation of slackwater deposits were located some 400 m up-tributaries rather than at the intersection of the mouths of present-day tributary junctions. These sites represent the intersection with tributaries during the extreme widths of floods on an alluvial floodplain. Physically tracing stackwater units up-tributaries was possible, in some cases 8 km. An accurate estimate of flood stage was determined by measuring up-terrace perpendicular to the tributary. During floods, aggradation and degradation of the main channel were insignificant because the broad floodplain is the most impacted feature in an alluvial setting. The preflood landscapes, determined from paleosol elevations, were recreated to estimate the Manning roughness coefficients and paleoflood discharges. The HEC-2 Water Surface Profile (Hydrologic Engineering Center, 1982) determined that paleodischarges of 4391 cm in 3590 BP and 8782 cm in 1150 BP emplaced the slackwater deposits along the tributaries of Black Bear Creek.