MODELING THERMAL GRADIENTS TO QUANTIFY GROUNDWATER - STREAM INTERACTIONS IN THE HEADWATERS OF THE WHITE RIVER: RESOURCE MANAGEMENT IN MANISTEE NATIONAL FOREST

Ecologically and economically important trout and salmon streams in the Manistee National Forest of Michigan are sustained by groundwater baseflow. In response to potential threats, largely from commercial groundwater extraction, National Forest resource managers seek to understand the dynamics of streamflow sustainability. The headwaters of nearly all ecologically significant streams in the forest are established in high permeability deglacial outwash sands. Distinct mechanisms of baseflow to streams include conduit style discharge points and diffuse seepage. In addition to piezometer nests and seepage meters, measurements and modeling of streambed temperature gradients may also help to characterize groundwater inflow to a headwater reach of the White River. Pore-water temperatures collected from three depths in the streambed during the fall/early winter of 2011 were modeled using the VFLUX program (Vertical Fluid [Heat] Transport Solver; Gordon et al, 2012). Those data generated suspect results, with calculated fluxes directed downward, and ranging from approximately 0 to 3 X 10^-5 m/s. Head measurements from two piezometer nests and a preliminary flux from a single seepage meter all generated vertically upward fluxes (consistent with field observations) of 2 X 10^-3 to 3 X 10^-8 m/s. An inversion of the SW – GW temperature gradient occurred early during the period of measurement in the modeled temperature data. During the growing season, surface water is warmer than groundwater, and during winter months the reverse is true. New time series data from the summer of 2012, with a constant downward temperature gradient, may permit us to evaluate the applicability of VFLUX thermal modeling to corroborate head measurements and seepage observations in these strongly gaining headwater stream reaches. Understanding the mechanisms of groundwater sustainability of stream flow provides forest managers the means to manage aquatic resources in responce to potential threats from development, and provides a characterization of these hydrologic systems in advance of potential climate change impacts.