IT’S COMPLEX: GROUNDWATER – SURFACE WATER INTERACTION IN HEADWATER STREAMS IN THE SOUTHERN APPALACHIANS

Groundwater and surface water interactions (GSI) in headwater regions of the southern Appalachians remain poorly understood, sometimes leading to over simplifications. Clarifying the type of GSI are important to an improved understanding of the evolution of stream water chemistry, groundwater recharge evaluation, the hyporheic zone, stream restoration, and management of riparian regions.

The traits of and controls on GSI were evaluated at three sites: two at hydrologic research stations established by the NC Department of Environmental Quality: Western Carolina Hydrological Research Station (WCHRS) (Cullowhee, NC), and Bent Creek Experimental Forest (Asheville, NC); and a third site in the Big Harris Creek watershed (near Shelby, NC)--part of a planned stream restoration project. All sites have groundwater wells; the WCHRS and Big Harris sites have stream gages. Hydraulic head, stream water temperature, and water chemistry data were used to characterize reaches for GSI.

The most consistent pattern in these headwaters systems is complexity. Over short reaches—even meters, streams can vary from gaining, to losing, to flow throw, to perched. Most headwater streams are minimally incised into a mix of bedrock, colluvium, and alluvium that favors complex GSI. For example, over a 600 m section of a 1^st order stream in the WCHRS, there are 4 distinct reaches: one strongly gaining, two losing, and one intermixed. At the WCHRS and Big Harris sites, post settlement deposition has altered riparian stratigraphy and, therefore, GSI. At the Bent Creek site, much of Boyd Branch (2^ndorder stream) is losing as groundwater flow is responding to a larger-scale topographic gradient feeding a trunk stream in the watershed.

At the WCHRS, headwater streams are mostly gravel bedded, but the gravel is largely a lag deposit resulting from the winnowing of fines from underlying colluvial materials with a low hydraulic conductivity. Consequently, valley side groundwater inputs to streams are largely lateral through stream banks. Reflecting a strong lateral connection, groundwater levels in riparian wells respond almost in sync with variations in stream discharge. Though complex, the patterns of the geomorphic, topographic, and stratigraphic settings are generally predictive of the style of GSI.