HYDROGEOMORPHIC INFLUENCES OF HYPORHEIC EXCHANGE IN A HEADWATER STREAM, CULLOWHEE, NC

Headwater streams make up the majority of the stream length in the U.S. and are a significant control on downstream water quality and biodiversity, especially so in the mountainous Southern Appalachians. The zone of interchange between surface and groundwater, the hyporheic zone, likewise exerts a strong control on stream health. The purpose of this study was to evaluate hydrogeomorphic controls that influence the hyporheic zone in headwater streams. The study area, the Gribble Gap catchment, is part of the Western Carolina Hydrologic Research Station (wchrs.wcu.edu); it has an area of 0.62 km² in a setting typical of the region.

To determine hydrogeomorphic controls in Gribble Gap, 11 stream-valley sections were characterized for numerous traits: side-slope angles; canopy; side-slope and riparian soils; stream bank height, bed material, shape, and width; and basic stream water chemistry (pH, temperature, conductivity). Analysis of these data showed differences in geomorphic setting and stream chemistry, which we interpreted to reflect differences in the hyporheic zone. Four representative sites were selected for detailed analysis of setting and water chemistry; 4-6 stream-bed and riparian groundwater wells were installed per site. Of the additional parameters evaluated, the stable isotope ratios, δ¹⁸O and δ D (n=49) were most informative.

The absolute difference between stream water and groundwater for both δ¹⁸O and δ D values was used as an indicator of mixing. Similar values of δ¹⁸O and δ D were interpreted to be well mixed and part of the hyporheic zone. From this, the extent of mixing was analyzed against the geomorphic traits. Of the traits evaluated, slope angle has a considerable effect (r² = 0.5 – 0.7) on the extent of mixing. Low slope angles correlate with a zone of well mixed stream water and groundwater, whereas poorly mixed waters were at stream sides adjacent to steep slopes. In many places, stream channel banks adjacent to low slopes are undercut with evidence of pipe flow, and are composed of porous gravels. The low slope angles coupled with porous channel banks promote active mixing of waters, thus a larger hyporheic zone. The results found here could be tested in other places and see if it extends to biotically important parameters.