THE ORIGIN OF GULLIES IN BEAVERDAM CREEK, NORTH CAROLINA

Urban development can strongly influence hydrologic and erosional processes in a landscape. In the Beaverdam Creek Watershed of North Carolina, several large gullies have eroded into the hillslope adjacent to the stream floodplain. We initially hypothesized two possible scenarios: 1) that road construction lead to increased overland flow culminating in erosion of the gullies in the hillslope or 2) that road construction lead to increased infiltration that resulted in throughflow along the soil/saprolite interface and sapping along the exposure of this boundary in the hillslope. In order to determine the origin of the gullies, we examined the sedimentology, soils, and geomorphic setting of a small field area in the vicinity of one of the gullies within the Beaverdam Watershed. Four soil pits were excavated and full soil descriptions according to Soil Survey Staff (1993) were completed for each pit. Pits excavated included a control pit on a non-eroded portion of the hill-top, two pits within the gully, and one in an alluvial fan directly below the gully. Organic carbon was analyzed for selected soil samples from these pits in the laboratory using loss on ignition procedures (LOI).

We found no evidence of significant overland flow in the upland surface of the gully. Nor did we observe evidence such as manganese concretions or mottling for perched sub-surface flow at the soil/saprolite interface that was exposed in the gully headwall. However, small groundwater piping features including ~ 0.2 m^3 sink holes are present at the base of the gully and are common along the length of the hillslope at the floodplain-hillslope contact. Lack of pedogenesis within the alluvial fan soil profile suggests that the gully is eroding rapidly, on a scale of several decades. Based on our observations, we suggest that the most likely candidate for gully formation is headward erosion initiated by piping at the base of the hillslope during episodes of high groundwater levels. Samples have been taken for Cesium-137 and Lead-210 in the alluvial fan pit and will be processed to further constrain the timing of gully formation. Whereas the presence of mature trees in the gully attests to some level of stability, the presence of other gullies in various stages of development along the hillslope suggest that gullying is an ongoing process that could be exacerbated by future development of the basin.