HYSTERESIS IN THE HYPORHEIC ZONE OF A REGULATED, LARGE COBBLE BED RIVER

Hysteretic relationships between physicochemical variables in surface water and vadose zone systems have been well studied. Published data suggest similar relationships exist within the boundary zone between groundwater and surface water (i.e., the hyporheic zone). It is likely that hyporheic hysteresis is controlled by a combination of surface water hydrology and the hydrogeologic properties of the riverbed, however most previous studies were conducted in small, natural streams with relatively static hydraulic controls. We studied the relationship between water quality, vertical hydraulic gradient, and changing river stage in the hyporheic zone of the Columbia River, a large, regulated, cobble bed river characterized by diurnal (approximately 2m) stage fluctuations. Our goals were to determine the nature of the hyporheic hysteresis and whether it was controlled by hydraulic properties of the riverbed.

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.