HYDROGEOMORPHIC CONTROLS ON THE EXPRESSION OF STREAM WATER IN <1 km2 PIEDMONT WATERSHEDS

Rapid development of the North Carolina Piedmont is converting headwater watersheds from forested or agricultural to urbanized landscapes, affecting the hydrology and geomorphology of small streams. We examine the water sources and contributing areas to headwater streams in 12 small, forested watersheds near the Charlotte metropolitan area. These watersheds have experienced a history of timber harvest and agriculture typical of Piedmont landscapes. Stream networks are characterized by regolith-bedded ephemeral channels that contribute to mixed bedrock and gravel-bed perennial channels. Source areas for ephemeral channels are on the order of 1 ha, while perennial flow heads have contributing areas on the order of 10 ha. Surface flow in ephemeral reaches occurs during rain events exceeding 2.5 cm and ceases within hours of rainfall. Between rain events, channel head locations vary by 0-14 m and correspond to bedrock exposures or soil pipes. Streams show varying patterns of baseflow discharge versus watershed area, with some streams showing evidence of concentrated zones of groundwater upwelling. Upwelling zones, characterized by temperature and conductance perturbations, are found in both bedrock and alluvial reaches and are stationary across seasons. Down-welling is observed in sediment wedges upstream of fallen logs or debris jams, sometimes leading to complete dewatering of surface baseflow. There are no consistent longitudinal trends in sediment size, and there is partial mobility of bed sediments under moderately-frequent flows. While the study watersheds represent pre-urbanization hydrogeomorphology, legacy land-use effects may contribute to variations in channel network extent and incision in the study watersheds.