DISCHARGE OF “MEGA-MEANDER” PALEOCHANNELS OF THE SOUTHEASTERN ATLANTIC COASTAL PLAIN, USA

Bankfull discharge was estimated for six large terminal Pleistocene meandering paleochannels (mega-meanders) in river valleys in southeastern Georgia and the Carolinas. Estimates were based on the slope-area method, and channel boundaries were surveyed from stratigraphic cross-sections constructed from closely spaced core and auger holes. Results indicate bankfull flood sizes that were similar to, or at most only modestly larger than, the discharge of low magnitude, high frequency floods on present-day Coastal Plain rivers. Previous mega-meander discharge estimates of two to four times modern values, calculated from planform dimensions, are likely overestimates. Mega-meander channels were wide and shallow, and their relatively modest discharge reflects their comparatively lower hydraulic efficiency relative to modern rivers, which resulted from a high width/depth ratio and a correspondingly reduced hydraulic radius. The exceptionally large planform dimensions of mega-meanders appear to be the product of a sediment discharge regime characterized by large quantities of bedload sand and relatively little vertical accretion. These factors influenced the composition of bed and bank material, resulting in a relatively high width/depth ratio. Regional increases in moisture indicated by pollen records for the southeastern Coastal Plain during the terminal Pleistocene provide a possible explanation for modestly increased discharge in cases where mega-meanders may have conveyed slightly larger than modern bankfull floods, but the precise climatic mechanisms that influenced precipitation delivery and runoff during this time remain poorly known. In the context of the late Quaternary evolution of southeastern Coastal Plain rivers, scrolled terminal Pleistocene mega-meanders likely represent a transitional meandering planform that was still influenced by large volumes of sandy bedload immediately following sand bed braiding of the late Wisconsin, ca. 30-17 ka. Findings illustrate the potential error of discharge estimates derived from planimetric channel dimensions. This has important implications for paleohydrologic studies, given that planform channel geometry is commonly used to estimate paleodischarge on Earth and in extraterrestrial settings.