THE TIMING OF AND CAUSE OF STREAM INCISION IN THE NORTH CAROLINA PIEDMONT

Incised streams throughout the Piedmont of North Carolina significantly impact valley bottom ecology and groundwater hydrology. Nonetheless, little has been to study the cause of incision and it is generally assumed that streams incised as a result of mill dam sedimentation as has been shown in the mid-Atlantic. In order to test this hypothesis, we mapped ~150 small dams over four counties in the central Piedmont of North Carolina. We examined the locations of 20 dams that were publicly accessible. At each site, we cleared stream banks and described the sedimentology both up stream and downstream of the presumed dam location. We additionally cleared and described stream banks at seven other incised locations not known to be associated with dams. In almost all locations, 1 – 1.5 meters of sandy legacy sediment overlies fine grained wetland deposits. Preliminary dating suggests that the legacy deposits coincide with early Euroamerican deforestation and gully formation on adjacent hillslopes. Field observations and LiDAR data show consistent incision both upstream and downstream of dam sites. As such, stream incision in the Piedmont is not consistent with dam sedimentation noted elsewhere. Instead, evidence suggests that incision was preceded by legacy sedimentation and then streams were often manually straightened by farmers and the Army Corps of Engineers. The remnants of dams typically only bridge the width of the incision indicating that they were built after the majority of incision was complete. The few reaches of stream that remain un-incised feature anastomosing morphology indicating that this may have been the dominant morphology in the region before Euroamerican settlers arrived.