NEW RADIOCARBON DATA ON FEATURE AGES, RIVER MIGRATION RATES, AND FLOODPLAIN SEDIMENTATION RATES IN CONGAREE NATIONAL PARK

The forested floodplain of the lower Congaree River Valley is underlain by a 4-mile wide complex of fluvial deposits that are Late Pleistocene to modern in age. These deposits contain abundant organic material including charcoal, seeds, and wood that can provide radiocarbon dates for reconstructing landscape development. Recent research has focused on sampling abandoned channels, levees, alluvial fans, and groundwater rimswamps using a Macaulay peat sampler and Eijkelkamp hand auger. Bulk sediment samples were wet-sieved and hand picked for dateable organic material. A few samples were collected from buried stumps exposed in cutbank outcrops during low water. Samples were then analyzed at the Center for Applied Isotope Studies at the University of Georgia. Here we present several new analyses as well as a few re-interpretations of selected existing dates.

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.