NEW RADIOCARBON DATA ON FEATURE AGES, RIVER MIGRATION RATES, AND FLOODPLAIN SEDIMENTATION RATES IN CONGAREE NATIONAL PARK
New data indicate that paleochannels associated with both Wise Lake and Tom’s Slough were abandoned and accumulating significant, organic-rich deposits by approximately 2,200 years before present (YBP, with “present” being 1950). This age is consistent with an earlier cutoff date of 2,860 YBP for Weston Lake Slough. Pairs of dates from organic-rich ox-bow lake deposits in Weston Lake Slough and Tom’s Slough indicate accumulation rates of 0.04 and 0.20 cm/yr respectively. Additional samples from muck swamp (a groundwater rimswamp developed above a Pleistocene ox-bow) and the Dry Branch alluvial fan are currently being analyzed. These data should help constrain the age of these features and chronology of the northern floodplain margin.
Overall the emerging dataset seems to suggest that Holocene Meanderbelt 4 of Shelley and Cohen (2007), which includes most of the visible abandoned channels in the lower valley, is less than 3,000 years old. Some groundwater rimswamp and alluvial fan deposits near the floodplain margin, however, hint at a deeper, late Pleistocene to early Holocene surface. Vertical accumulation rates near the river have apparently accelerated in historical times. While this increase is largely attributed to legacy sediment from historical landuse upstream, this signal is complicated by an apparent natural increase caused by lateral migration of rapidly-accumulating levees over deeper, more slowly-accumulating backswamp deposits.