Laserfiche WebLink
• ALLWIAL AQUIFERS <br />A description of the alluvial aquifers is contained in a report <br />by Metcalf & Eddy (1975). The unconsolidated alluvium that partly <br />fills the bottom of Davis Gulch and Lower Middle Fork Valley con- <br />tains and transmits groundwater, but the storage capacity and <br />sustained yields to wells are small at 10 to 30 gpm. The alluvium <br />in Upper Middle Fork Valley contains practically no groundwater. <br />Permeability of the alluvium generally is high; thus it functions <br />chiefly as a recharge medium for underlying bedrock. The ground- <br />water in the alluvium moves laterally down-valley above the basement <br />bedrock, and some moves downward into fractures in the bedrock. <br />During spring snowmelt, and for a short period thereafter, the <br />alluvium in the two main gulches receives a contribution of ground- <br />water by recharge from stream infiltration and from the intermit- <br />tent streams and alluvium in the tributary valleys. The thickness <br />and storage capacity of the alluvial fill in these tributary valleys <br />• is small, so the groundwater contribution from them is of short <br />duration, probably for only 1-1/2 or 2 months after the beginning <br />of recharge from snowmelt. <br />When the stream levels drop after the peak surface discharge of <br />Davis and Middle Fork creeks during the spring, groundwater from <br />bank storage in the alluvium enters the streams. However, during <br />the greater part of the year, except during spring runoff and <br />rainstorms, most of the flow of both creeks is maintained by the <br />combined discharge of springs. Most of the springs issue from <br />the bedrock and enter along the edges or in the beds of the streams. <br />The component of groundwater remaining in the alluvium, after <br />some has percolated downward into the bedrock, surfaces and enters <br />the streams where the alluvium is absent above both Davis and <br />Middle Fork falls. Groundwater in the alluvial fill below both <br />falls originates from stream water which percolates into the allu- <br />vium, and then moves down-valley and beneath the southern property <br />boundary as subsurface outflow. In this reach of the valley, <br />G-20 <br />