HYSTERESIS IN THE HYPORHEIC ZONE OF A REGULATED, LARGE COBBLE BED RIVER
We monitored hyporheic water quality during periods of fluctuating river stage, and determined that although hyporheic temperature, specific conductance, and dissolved oxygen co-varied with river stage [p<<0.05], there was a lag in their response, suggesting a hysteretic relationship. Additionally, we monitored the vertical hydraulic gradient (VHG) in the hyporheic zone and determined that it too was governed by a hysteretic relationship with river stage. The nature of the hysteresis varied at different locations, despite equivalent stage fluctuations, indicating that additional controls were present. In order to determine whether river bed hydraulic conductivity (K) was a control, we compared K to the VHG at three locations within the hyporheic zone. The VHG hysteresis showed the greatest response (i.e., largest range) to river stage at the least permeable location (K=2.9 x 10-4 cms-1) and showed the least response (i.e., smallest range) at the most permeable location (K=8.0 x 10-3 cms-1). Our results emphasize the need for regulators to consider surface water fluctuations and river bed permeability when attempting to collect representative hyporheic water samples.