SENSITIVITY OF HYPORHEIC ZONE SEDIMENTS TO GEOMORPHIC DISTURBANCES IN A KARST WATERSHED IN EASTERN KENTUCKY

The alluvial sediments comprising the hyporheic zone serve as important habitat for aquatic vertebrate and macroinvertebrate fauna. Many of these organisms prefer to spend significant portions of their life history living below the armor layer of the stream or deeper in the stream substrate. In karst settings stygobites (obligate cave dwellers) have been observed to colonize the hyporheic habitat of surface streams adjacent to cave environments. This behavior is likely due to the similarities between deep cave and hyporheic zone environments. However, the dynamics of sediment mobilization and other basic geomorphic processes in karst streams have received limited investigation.

To evaluate the stability of these habitats and assess the frequency with which they are disturbed Cave Branch stream in eastern Kentucky was investigated. Along its course in the study area Cave Branch alternates from a surface stream to a cave stream several times. Nineteen detailed stream cross sections were measured in Cave Branch and then tied together by a longitudinal survey of the stream. Sediment size, stream velocity, and local gradient data were also collected at each site. A comparison of basal shear stress and critical shear stress is being used to determine the dynamics of sediment mobility at various stream stages.

Preliminary data analysis indicates that the ratio between basal and critical shear stresses for the mean particle sizes approaches or exceeds 1.0 (range: 0.97 – 8.3) at stages that are equivalent to 75-90% of bankfull discharge. At bankfull discharge the stream is able to entrain the largest particles observed in its bedload. If the frequency of bankfull discharge in karst streams is similar to that in non-karst streams it is likely that hyporheic communities are adapted to frequent and large-scale fluvial disturbances.