VARIATION OF HYPORHEIC ZONE CHARACTERISTICS BENEATH A MICHIGAN FRESHWATER STREAM

The exchange of groundwater and surface water is an important process contributing to the health of streams and watersheds. In this study, we set out to characterize the Hyporheic Zone (HZ) within a small southeast Michigan Stream. Our goal was to measure physical and chemical characteristics that would delineate the HZ extent and quantify the driving forces for nutrient fluxes through the HZ. We hypothesized that we could differentiate the HZ from adjacent groundwater and surface water environments using multiple components of the system: stream morphology, substrate particle size, water movement, water chemistry, and resident biota.

The stream is a tributary of the Clinton River that is continuously fed by a retention basin which collects storm water runoff from surrounding subdivisions within Rochester Hills, Michigan. The basin outfall grate regulates normal stream flow volume. The stream has steep banks that are heavily undercut, however, attesting to periodic storm flow events. The river bottom along the study site consists of up to one meter of sand and gravel. This substratum provides adequate pore space for groundwater and surface water to interact and develop a mixing layer. An underlying silt (diamicton) aquitard adds a component of variation to the stream hydrology, however. The top of the aquitard layer appears to be an erosional surface that rises above the stream level at several locations along the studied transect. Thus sediment distribution governs the movement of water between the subsurface and riverbed interface.

Four mini-piezometer transects identified intervals of gaining and losing stream reaches along two riffle-pool sequences. Fluctuation between gaining and losing conditions was observed in several piezometers. The fluctuation suggests either temporal variation in water movement between the stream and stream bed, or that the mini-piezometers were initially in disequilibrium. Evaluation of water chemistry from three transects of two-inch monitoring wells installed at depths of 10, 20 and 30 cm is ongoing. The final stage of investigation will evaluate the taxonomic diversity of macroinvertebrates within the system. Colonization cores will be used to sample epigean and hypogean species abundance and community structure.