INTERPRETING HYPORHEIC EXCHANGE FLOWS AMIDST A MOSAIC OF STREAM-CATCHMENT CONNECTIONS

Interpretation of hyporheic exchange flows (HEFs) has progressed in the past 25 years from identifying individual HEFs to understanding the broader hydrologic context. Stream studies (with several colleagues) in Colorado (Mineral Creek, Silverton, Colo.) and Montana (Stringer Creek, Tenderfoot Experimental Forest, Montana) document stream-catchment connections of which hyporheic exchange flows are a component. These flows, typically studied at the 10-meter scale, occur amidst a mosaic of stream-catchment connections at both finer and coarser scales. In Mineral Creek, detailed tracer sampling shows that HEFs occur in a hydro-geochemical setting of significant meter-scale variation in the concentrations of acid mine drainage constituents. Transient Storage Model (TSM) simulations with the USGS code OTIS [http://co.water.usgs.gov/otis/] have been used in numerous streams to interpret in-stream responses of solute transport for streams influenced by HEFs [for examples, see http://smig.usgs.gov/SMIG/transtor_reader1.html]. In Stringer Creek, OTIS simulations are interpreted to suggest HEFs occur in a hydro-solute transport setting of continuing inflows and outflows within 100-200 m sub-reaches along the nominally gaining stream. Challenges for understanding hyporheic exchange flows, as addressed by several investigators, are further enhancing the usefulness of TSM simulations. In addition, a variety of process-based tools are simultaneously used, again by several investigators, to develop understanding of the hydrologic mechanisms by which streams are connected with their catchments.