HYPORHEIC INTERACTION ALONG A SEMI-ARID SECOND ORDER STREAM, WIND RIVER RANGE (WY)

We report the results of chloride solute tracer injection experiments conducted during July 2003 to evaluate hyporheic interaction along a 300-m reach of Red Canyon Creek, a second order stream in the semi-arid Wind River Range of Wyoming. The study site is part of a watershed managed by The Nature Conservancy of Wyoming and used as a hydrologic field site by the University of Missouri Branson Geologic Field Station. During our experiment, the stream had a discharge rate of 6 ft³/s and a chloride concentration of ~2.2 mg/l. Chloride brine at ~120,000 mg/l was injected into the channel at ~1.0 l/min for two hours and water samples were periodically collected at four locations, between which were channel features likely to enhance water storage; including beaver dams, severe meanders, and pool-riffle sequences. From the shape of the Cl^- concentration versus time curves at each observation location, we inferred the hyporheic characteristics of water storage between the injection and observation sites. We also used a one-dimensional surface water solute transport model (OTIS) to inversely model storage parameters by statistically optimizing them to create a best-fit model based on experimental data. The values for solute transport parameters, including the storage exchange rate (a) and the storage zone cross-sectional area (A_s), were used to determine the flux of water through storage zones, the average residence time of water in storage zones, and the average distance traveled by a water molecule before entering a storage zone.

Our results show that although the stream has relatively limited local hyporheic interaction resulting from the presence of debris dams, there are large-scale hyporheic flowpaths around meander bends. Potentiometric surface and water table maps prepared from water table monitoring wells and piezometers installed in the study area suggest there is large-scale hyporheic mixing along flow paths several meters long. While our tests were done towards the end of winter snowmelt recession, we anticipate that stream tracer tests would better capture the extent of hyporheic interaction during periods of low flow during early fall. To capture this scale of hyporheic mixing, stream tracer tests will be next done in late summer for longer periods of time and coupled with observations at near-stream wells on-site.