DAMS, FARMERS, AND LEGACY SEDIMENTS: THE MESSY DRIVERS OF STREAM INCISION IN THE NORTH CAROLINA PIEDMONT

Streams in the North Carolina Piedmont are consistently incised between 1 and 3 meters. Over the last decade, it has often been assumed that this incision was related to mill dams that existed throughout the area. However, after mapping ~150 dams over four Piedmont counties, we found that incision above dams is roughly equal to incision below dams. In fact, most dams that survive are only as wide as the incised channel suggesting that incision predates dam construction. Instead, incision in the region appears to be much more complicated. Recent dating of gullies in the area suggests that anthropogenic erosion in the region began much earlier than previously thought. Specifically, it appears that upland erosion began immediately after Euroamerican settlers arrived in the late 18^th century and deforested the area. Sedimentology from streambanks indicates that the sediment from this upland erosion was deposited in the valley bottoms as streams were unable to transport the influx of sediment. Then, as uplands stabilized via agriculture and subsequent abandonment of farmland, streams would have incised through the legacy sediments covering the floodplain. Further, aerial photographs from the early 20^th century suggest that farmers, and later the Army Corps of Engineers, aggressively straightened streams so that wet valley bottoms could be farmed. In some areas, incision continued down through the legacy sediments, through underlying floodplain deposits, and into the soft, underlying saprolite. This new base level was transmitted headwardly through the tributary network until the majority of streams in the region were incised. The result is that only streams that were protected by erosion resistant rock or human infrastructure were protected from incision.