HILLSLOPE SOIL AND HILLSLOPE SEDIMENT DEVELOPMENT AND MOBILIZATION IN TWO LOW-ORDER WATERSHEDS IN THE SE PIEDMONT, NC USA

Despite a long history of soil and geomorphic research in the SE Piedmont focused on sediment accumulation in valley bottoms, there are few studies of the nature and dynamics of Holocene hillslope soils and hillslope sediments in low-order watersheds that make up most of the region.

This study characterizes four toposequences - including adjacent valley deposits - in two first-order watersheds (4.5 and 6.2 ha) of the SE Piedmont where the entire Holocene source-to-sink system is still intact. The watersheds are underlain with similar lithologies (felsic metavolcanics) but vary in aspect (N & S) and in proximity to the major, high-order river. Soil profiles and sediment sequences were described for 5 soil pits (~1.5 m deep) per hillslope toposequence. Samples (n=146) from all 21 soil pits were analyzed for particle size, bulk elemental content, carbon content, pedogenic iron content, and radiocarbon age where available.

Overall, north-facing slopes and watersheds more distal from major rivers exhibit more mature soils forming in deeper sediment. Both exhibit higher clay content, lower pedogenic iron ratios, less sand, and higher organic content than their south-facing or river proximal counterparts. Hillslopes also vary in their contribution of sediment to adjacent valley bottoms. North-facing and river proximal hillslopes exhibit more and thicker buried soils with higher values of chemical enrichment for Si/Al than their south-facing counterparts. Clay and silt content are also highest in summit and shoulders of interfluves, and sand and gravel content increase towards the valley bottoms as hillslope sediments thicken. Thus, we find soil development and sediment mobilization on Piedmont hillslopes to vary not only by aspect, but also by hillslope position within Piedmont watersheds, even for hillslopes adjacent to streams of similar orders. The implications of these observations prompt a reevaluation of Piedmont soils, suggesting their formation histories are polygenic in nature.