COMPLEX GLACIOFLUVIAL STRATIGRAPHIC CONTROLS ON GROUND WATER AND SURFACE WATER IN HORSESHOE LAKE, KING COUNTY, WASHINGTON

Horseshoe Lake is a small lake formed in a closed topographic depression in southeast King County. The lake has a history of dramatic surface water level fluctuation, up to 20 feet. Surface water flooding of surrounding residential property occurred in 1991 and 1996, and more recently threatened homes in early 2007 and early 2008. Geologic and hydrogeologic information obtained from subsurface exploration, geologic mapping and water level monitoring in the Horseshoe Lake subbasin have been integrated to develop a conceptual hydrogeologic model that provides context for the dramatic surface water level fluctuations.

The geomorphology in the vicinity of Horseshoe Lake is largely the result of processes associated with the most recent glacial episode, the Vashon Stade of the Fraser Glaciation. The Horseshoe Lake topographic depression was formed by meltwater streams emanating from the base of the Vashon glacier as the ice retreated northward during deglaciation. The high-energy meltwater streams flowed across a layer of low-permeability Vashon lodgement till that had been deposited at the base of the Vashon glacier. Detailed subsurface geologic exploration data verifies that in some areas, the recessional meltwater streams eroded “windows” through the basal Vashon lodgement till, and into underlying pre-Olympia-age fluvial deposits. This conceptual hydrogeologic model is fundamentally different than previous ground water studies of Horseshoe Lake, which modeled the lake system as perched above low-permeability Vashon lodgement till.

Ground and surface water monitoring data also demonstrate Horseshoe Lake is not formed in a till-bottomed depression. Since the till is missing there is a direct hydraulic connection between Horseshoe Lake and deeper ground water. Hydrographs of Horseshoe Lake surface water and pre-Olympia aquifer water levels illustrate similar timing and magnitude of response to precipitation. Horseshoe Lake water elevations are higher, indicating vertical flow from the lake into the pre-Olympia aquifer. Based on ground water elevation data from multiple monitoring wells, ground water flow in the pre-Olympia aquifer interval is primarily toward the southwest, and flows to springs within Crisp Creek and along the northern wall of the Green River Valley.