GROUNDWATER SUSTAINABILITY OF STREAMFLOW BY DISTINCT MODES OF STREAMBED SEEPAGE IN THE WHITE RIVER, MANISTEE NATIONAL FOREST, MICHIGAN

Preliminary findings suggest at least three distinct mechanisms of groundwater discharge to a headwater reach of the White River in Michigan are operating to sustain stream discharge. These distinct modes of groundwater seepage occur over spatial scales on the order of meters, operate in apparently homogeneous unconsolidated glacial sands, and differ by orders of magnitude in their contribution to streamflow. The White River, a Michigan State Natural River and candidate Federal Wild and Scenic River, is a groundwater baseflow dominated stream that forms a significant trout and salmon habitat. Cool stable temperatures and stream discharges are prerequisite for its critically important aquatic ecosystem services.

Diffuse, and relatively low-magnitude streambed seepage has been measured near channel center with a seepage meter. Hydraulic gradients, and calculated groundwater discharge, are higher in the streambank-marginal littoral area. Finally, scattered throughout the channel, littoral, and streambank areas are discrete sites of focused, high-energy, conduit-style groundwater discharge points. Measured values suggest these individual sites, while few in number relative to channel area, may provide a significant proportion of stream discharge over time. Slug test results and laboratory permeameter measurements of the shallow riparian aquifer sands and streambed sands are uniform (10^-3 cm/s) and permit flux calculations at sites with differing hydraulic gradients. It is possible to develop a conceptual model considering an estimated contributing area for each of these distinct modes of groundwater discharge that fully defines stream discharge. Further, at the high and moderate discharge sites, streambed groundwater at 0.5 m depth displays distinctly different temperatures as a result of different hydraulic gradients, indicating variable thicknesses of hyporheic zones.