EROSION AND DEPOSITION IN A DISTURBED WATERSHED: GULLIES, FANS AND TUNNEL FEATURES

Increased understanding of past erosional and depositional processes in the Southern Appalachians caused by climatic and anthropogenic land disturbances is needed to characterize transfer of sediment and its hydrological impact today. Gullies, alluvial/colluvial fans, and stream tunnels were studied in the Gribble Gap catchment (0.4 km²) located in the headwaters of the Little Tennessee River in the mountains of western North Carolina. The study area experienced debris flow deposition like much of the surrounding region, as well as logging and conversion of woodlands to pasture in the late 1800s through much of the 1900s, which lead to soil erosion.

Five gullies were surveyed in Gribble Gap to calculate volume loss. Bulk density measurements from soil pits in the gully walls were used to convert volume to mass. Four ~1 m-deep soil pits were dug in fans and colluvial/alluvial deposits and breakability of clasts, soil texture, and Munsell colors were used to provide relative fan ages. Charcoal was collected to provide an absolute date of a layer 80-87 cm below the fan surface. Soil pipes, which provided flowpaths through fan deposits, were characterized by measuring a series of pipe dimensions and vertical position relative to the first stream terrace. Temperature probes and wells were also installed to determine if the pipes were gaining or losing groundwater.

The preliminary estimate of total volume of gully erosion that followed clearing was calculated to be 2893.6 m³ of sediment from a 0.14-hectare area. Downstream of one of the gullies, a debris flow deposit was found to be very poorly sorted with clasts of gneiss and quartz varying from cobbles to boulders. Relative dating indicated the fan is likely from the early Holocene or late Pleistocene. Charcoal dating from the fan yields a date of 1030 – 1220 AD; however, whether the carbon was transported or formed by burning in situ is unclear. A gravel layer at approximately 0.95 m below a stream terrace overlain by a layer of finer sediment is associated with the pipes. The pipes are losing reaches for groundwater, as indicated from well and temperature data. While the erosional impact of gullies on the system has been quantified, differentiating older debris flow deposits from younger, likely alluvial, deposits associated with human activities has thus far been elusive.