MULTI-SCALE STREAM TRACER TEST IN LOOKOUT CREEK, OREGON: FRACTAL BEHAVIOR OF HYPORHEIC RESIDENCE TIME DISTRIBUTIONS

The primary goal of this project was to quantify the temporal scales of hyporheic exchange along a stream network and to examine how hyporheic exchange scales with increasing stream size. Many previous studies focused on single stream reaches, or on several reaches of similar sized streams, whereas we examined the residence time of water in the hyporheic zone over increasing length scales in a single stream. Prior work based on tracer data and analysis using transient storage models with exponential residence time distributions (RTD) suggests that hyporheic parameters - primarily volume and mean residence time – are both spatially and temporally scale-dependent. However, recent work hypothesizes that the scale dependence may be partially addressed by using a power-law RTD (Haggerty et al., 2002). Our research tested this hypothesis by conducting several multi-scale tracer tests in a single stream.

A single, multi-scale tracer test was conducted in Lookout Creek, Oregon, to test the power-law RTD model and to examine hyporheic scaling relationships along the length of the stream – from its 2nd-order headwaters through its 5th-order mainstem. The longitudinal tracer test involved an in-stream injection of Rhodamine WT over 78 hr, followed by monitoring at seven downstream locations for five months. Three additional inter-order tracer tests were carried out, between each of the major stream confluences along Lookout Creek, to quantify the behavior of the hyporheic RTD and to separate geomorphic variability from other possible scale-dependent factors. The results from the longitudinal and inter-order tracer tests clearly demonstrate power-law RTDs, but not a “universal” (basin-wide) RTD. The slope of the late-time tail in tracer concentrations which characterizes the RTD, decreases downstream (or with increasing stream size). The observed changes in the RTD may result from characteristic changes in channel morphology or the degree of geomorphic complexity which are related to stream size. Larger streams exhibit a slightly longer delay before assuming a power-law distribution when compared to the smaller streams.