Streambed exchanges along tributary streams in humid watersheds

Tributary streams flow from the headwaters directly to the main stem in the valley axis and represent an important general stream type with high surface areas to volume and high streambed exchanges along much of their length. These characteristics strongly influence water quantity and quality in tributary streams, as well as the underlying groundwater and, eventually, the main stem. Tributary streams can be described as consisting of upland, central, and distal reaches, each with temporal trends in streambed exchanges unique to their spatial position along the tributary stream. In this work, thermal tracing and hydraulic monitoring reported in earlier works have been analyzed to compare surface-water/groundwater streambed exchange patterns along Fish Creek Wyoming (US), a well-studied tributary stream in a humid watershed. Results of this analysis demonstrate that composite thermal/hydraulic techniques successfully differentiated reach-specific streambed exchanges to distinguish upland, central, and distal reaches along Fish Creek above the Snake River. The upland and central reaches streambed exchanges were primarily vertical and horizontal streambed exchanges normal to general streamflow, while the distal reach streambed exchanges were lower in magnitude and more longitudinal in the same direction as streamflow. Heat-based streambed hydraulic conductivities were highest (and isotropic) for the upland reach and lowest (and strongly anisotropic) for the distal reach, in accordance with general downstream sediment fining approaching a main stem. These distinct reach patterns should show general transfer value, since general tributary streambed exchange patterns are expected to be influenced by proximity to headwaters in the upland reach, by buffering within alluvial fill in the central reach, and by proximity to the main stem in the distal reach.