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. Parachute Creek Member: This water-bearing unit is the most <br />productive aquifer within the site area. The member's highest <br />permeabilities and groundwater production potentials are in <br />the major fracture systems beneath Davis Gulch and the valley <br />of the upper part of Middle Fork. The shale that comprises <br />the member is dense and does not transmit groundwater through <br />pore spaces as would an aquifer such as a sandstone unit. <br />Groundwater movement and storage in the Parachute Creek member <br />is in the fracture systems, joints, and openings along bedding <br />planes, which provide access for recharge and conduits for <br />vertical and horizontal flow. The fracture and joint systems <br />control the major quantity of groundwater flow. Wells drilled <br />into this member yielded groundwater at a rate of 100 to 300 <br />gpm. Since the shale of the Parachute Creek member is practically <br />impermeable, except where it is fractured or jointed, recharge <br />occurs through the fracture zones. Along Lower Davis and Middle <br />Fork gulches above the falls where the bedrock is exposed, <br />• recharge occurs directly into the fractures. On the steep <br />bedrock cliffs on both sides of Davis and Middle Fork falls <br />and Lower Middle Fork Creek, overland runoff is so rapid that <br />practically no water reaches the water table in the bedrock <br />(Metcalf ~ Eddy, 1975). <br />Evacuation Creek Member: This member is primarily silty sandstone <br />and in most locations lies above the zone of groundwater saturation. <br />This unit is topographically high and caps the plateau. Within <br />the project site the unit is mostly drained of groundwater. <br />It functions chiefly as a recharge medium for the underlying <br />Parachute Creek member during spring snowmelts and periods <br />of precipitation. Groundwater moves through the fractures <br />and vertically and laterally through the beds of sandstone <br />and siltstone. <br />Movement is slow because (1) the fracturing is not extensive, <br />and (2) the beds of sandstone and siltstone are fine grained <br />• <br />G-22 <br />