COMPLEX GROUNDWATER – STREAM WATER INTERACTIONS IN A HEADWATERS CATCHMENT, WESTERN NORTH CAROLINA

Headwater streams exert strong control on downstream discharge, water quality, and ecology. The focus of our study, the Gribble Gap basin (0.4 km²), is located in the headwaters of the Western Carolina Hydrologic Research Station. Previous research in Gribble Gap has shown the streams in the catchment include gaining and losing reaches with variable chemistry. The general goal of this study was to understand these differences. The specific study objectives were to learn why reaches vary between gaining and losing, to quantify the linkage between stream and groundwater, and to discern likely groundwater flow paths feeding gaining reaches.

To understand geologic controls on gaining and losing reaches, riparian and in-stream wells were sampled for head levels over time, and sediments were analyzed for texture. Thermal traits of water were used to identify gaining reaches, which focused water-sampling efforts. Headwaters of tributaries were measured for conductivity to infer depth of flow paths of source groundwaters. Last, experimentally, surface flow of a small stream, Wind, was diverted for 75 m. This permitted quantitative and qualitative observations of new, emerging groundwater in the ‘dried’ reach.

Collectively, the data indicate high complexity in the catchment. Water conductivity at the heads of tributaries varies 3 fold, likely indicating deeper groundwater flow sources for northern tributaries. Water chemistry was similar in gaining and losing reaches, but losing reaches had greater temporal variation than gaining. Sediment analysis shows losing reaches, sometimes disconnected from the underlying water table, are immediately underlain by silt-clay rich sediments that transition to sandy sediments with high hydraulic conductivities (<0.8 cm s^-1). The experimental diversion of surface water caused a rapid decline in water levels of stream-side wells, up to 10 cm, indicating a very direct connection of surface and groundwater. In sum, there appear to be large-scale groundwater flow differences across the catchment while local variation in groundwater-surface water interaction is controlled by geomorphic and sediment traits. The Gribble Gap basin has traits common to the southern Appalachians, so observations in this study are likely predictive of many headwater catchments.