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The uppermost hydrologic unit in most of the Stephens Creek is the older rock aquifer ( McFarland and Morgan, Plate 2, 1996). Specific depth to groundwater data is not available. However, generally depth to groundwater increases with increasing elevation. In the Willamette watershed planning area, perched groundwater exists throughout the upland areas in the west-side subwatersheds.
Generally, perched groundwater exists above a fragipan layer between 2.5 and 4.5 feet below ground. This shallow perched groundwater can create natural hazards, particularly when tapped or daylighted by road or building cuts. It can precipitate landslides and cause soil creep, with potentially serious consequences for development. Groundwater is also susceptible to pollution for a variety of sources, including septic drainfield effluent, pesticides, herbicides, fertilizers, solid waste leachate, and runoff from parking lots and other impervious surfaces ( City of Portland Bureau of Planning, 1991 and 1995).
McFarland and Morgan (1996) estimated groundwater recharge throughout the Portland Basin. Generally, infiltration in the Tualatin Mountains in this subwatershed ranged from 20 to 25 inches annually ( McFarland and Morgan, 1996).
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