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The uppermost hydrologic unit in the lower terrace portion of the Miller Creek subwatershed is the unconsolidated sedimentary aquifer. The groundwater level is less than 50 feet above mean seal level (msl). The thickness of the aquifer ranges from zero to 199 feet. The unconsolidated sedimentary aquifer ends roughly at the base of the Tualatin Mountains where the older rock aquifer becomes the uppermost hydrologic unit ( McFarland and Morgan, Plate 2, 1996). Underlying the unconsolidated sedimentary aquifer in the lower terrace portion is the Troutdale gravel aquifer, which begins less than 20 feet above msl and is up to 199 feet thick. Immediately west of Highway 30, depth to groundwater ranges from 150 to 400 feet.
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