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