HYDROGEOLOGY OF THE EASTERN BEAR CREEK PLATEAU, KING COUNTY, WASHINGTON

Ground water flow in the Vashon advance aquifer beneath the eastern margin of the Bear Creek Plateau is controlled by the location and distribution of north-south trending pre-Vashon age paleotopographic highs, and intraformational stratigraphic discontinuities. Incipient drainage development on the Olympia-age depositional surface formed in response to a lowering of base level associated with a relative drop in sea level prior to the onset of the Frasier Glaciation. Thin interval thicknesses of Vashon glaciofluvial sediments reflect deposition on the paleotopographic highs. Thick sequences of glaciofluvial sediments are located in paleotopographic drainages.

Ground water flow patterns in the Vashon advance aquifer are constrained by the north-south trending paleotopographic high located along the extreme eastern margin of the Bear Creek Plateau. The paleotopographic high is dissected by an east-west trending paleovalley. Ground water converges from the north and south into the paleovalley, and discharges at a rate of about 1 cubic foot per second forming the largest spring system along the eastern margin of the Plateau for several miles.

Water level monitoring and aquifer testing indicate the aquifer exhibits unconfined and leaky confined behavior at different locations in the aquifer system. Transmissivity of the aquifer ranges from approximately 150 to 15,000 feet squared per day. The lowest transmissivity values were calculated from aquifer tests performed on wells located on the paleotopographic high where the aquifer was relatively thin. Calculated storage coefficients range from 0.03 to 0.005.