Using Multiple Watershed-scale Dye Tracing Tests to Study Water and Solute Transport in Naturally Obstructed Stream Channels

Temporary storage of surface water at channel sides and pools significantly affects water and solute transport downstream in watersheds. Beavers, natural "stream channel engineers", build dams which obstruct stream flow and temporarily store water in small to large ponds within stream channels. These ponds substantially delay water movement and increase the water residence time in the system. To study how water and solutes move through these obstructed stream channels, we did multiple dye tracing tests at Cherry Creek, a main tributary to Red Canyon Creek (Wind River Range, Wyoming). First we surveyed beaver dam distributions in detail within the study reaches. We then introduced dyes four times from July 2nd to 6th, 2007 using a scale-up approach. The observation site was fixed at the mouth of Cherry Creek, and 1.5 grams of Rhodamine WT (RWT) dye was injected sequentially at upstream sites with increasing test reach length. The reach lengths scaled up from 500m to 2.5 km. A field fluorometer recorded RWT concentrations every 15 seconds. The results show non-linear decreases of the peak concentration of the dye tracing cloud as the reach scaled up. Also, the times to 1.) the arrivals of the leading edges (Tl), 2.) the peak concentrations (Tp) and 3.) the tailing edges (Tt) and 4) the durations of the tracer cloud (Td) behaved non-linearly as function of length scale. For example, plots of arrivals of leading edges and tailing edges with scale distance appear to define curves of the form; Tl=27.665e1.07× Distance (r2=0.99) and Tt=162.62e0.8551× Distance (r2=0.99), respectively. The greatest non-linearity occurred for the time of tailing and the least for the time of leading edge. These observations are consistent with what would be expected with greater density of dams and/or storage volumes as the reach length increased upgradient. To come to a first approximation, we are currently modeling the breakthrough curves with the solute transport code OTIS to address the relative differences in average travel velocity, longitudinal dispersion, and storage parameters from the mouth to the headwaters of the creek.