VARIABILITY OF THE CONGAREE RIVER FLOODPLAIN MARGINS: CLUES TO ORIGIN, GEOMORPHOLOGY, AND GEOHYDROLOGY

The lower Congaree River Valley (CRV) is an asymmetrical feature nested into a regional sequence of Upper Coastal Plain (UCP) strata tilted gently south across the valley axis. New mapping in the modern floodplain (FP), home to Congaree National Park, has revealed significant differences between the northern and southern margins with respect to age, origin, and ongoing geomorphic and geohydrologic processes.

The FP is bounded to the north by Pleistocene (>100ka?) fluvial terrace deposits overlying saturated Cretaceous deposits. To the west, the northern margin is defined by a series of embayments cut by meander scars dating back at least 21 ka. Scarp expression is <15', and commonly reduced by colluviation, minor alluvial fan development, and FP sedimentation. Rimswamps are locally developed where groundwater seeps from the bluffs near old channel axes, but the ridge scroll complexes inboard often represent a second bottoms characterized by dry “islands” of flood intolerant vegetation flanked by well-developed streams. Drainage patterns suggest that these islands are formed by depression of the groundwater table in the vicinity of Cedar Creek (CC), a major tributary locally incised ~6' into the FP. Incision is potentially tied to historical land use changes in the area and a large dam immediately upstream on CC. To the east, a large remnant of a Pleistocene braided terrace is present along the northern FP margin. Similar terraces have been described from GA to NC and dated at up to 60 ka. This terrace, which is also a second bottoms, is cut by younger meander scars within the FP.

The modern river flows within a distinct meanderbelt adjacent to the southern FP margin, which is a steep bluff cut into semi-consolidated Late Cretaceous to Eocene UCP deposits. Immediate relief is locally sub-vertical and <200'; total relief is >350'. This margin represents a compound surface resulting from progressive southward migration and incision (combing) of the entire CRV from the Middle Pliocene (~3.5 ma) to recent. Late Pleistocene dune complexes on the southern FP indicate that this margin was cut by that time, and has been re-occupied by the modern river only recently. These bluffs are subject to retreat through undercutting by the modern river, mass wasting controlled by failure of claystone and iron-oxide-cemented sandstone beds in the bluffs, and significant tributary incision.