MULTIPLE SPATIAL SCALES OF SURFACE WATER-GROUNDWATER EXCHANGE IN A HEADWATER STREAM IN MONTANA, USA

Reach-scale solute transport studies are often used to characterize transient storage of solutes or groundwater-surface water exchanges, yet the results characterize only a small component of a longer stream. We examined the spatial distribution of streamflow exchanges in a headwater stream at the US Forest Service's Tenderfoot Creek Experimental Forest in central Montana, USA. In early August 2005, we conducted a long-term (approx. 8 days) Rhodamine WT (RWT) tracer injection in a 2.8 km stream reach. When steady-state RWT concentrations were achieved, we then conducted a series of chloride slug (approx. instantaneous) releases in 28 100-m and 14 200-m long subreaches along the 2.8-km study reach. Stream discharge increased from 0.8 L s^-1 (at the head) to 27 L s^-1 (at the base) along the 2.8-km study reach. Thus, although the stream gains water at the scale of the 2.8-km study reach, results of these tracer experiments demonstrated simultaneous hydrologic gains and losses in the individual subreaches on multiple spatial scales. Subreaches with some of the larger losses of water were also the subreaches with larger gains of water. Simultaneous gains and losses measured within a given subreach are consistent with longer subsurface, hyporheic, hydrologic retention times not measurable through interpretation of recovered tracer alone. In all 100 m and 200 m subreaches, mass loss of RWT from the long-term injection was less than that of chloride from the slug releases, indicating the influence of long hyporheic flowpaths along the 2.8 km reach.