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of up to 300 feet of the superimposed spoil fill. It is also possible that some subsidence <br />will occur in the vicinity of the old underground mine workings on the north side of <br />the valley as a result of the weight of the embankment. Because of these foundation <br />conditions and the likelihood of future settlement taking place, the original scheme <br />of conveying the occasional runoff in Streeter Gulch through a conduit under the em- <br />bankment was abandoned. <br />Hydrologic Considerations and Drainage <br />The natural drainage area lying above the mouth of Streeter Gulch is approxi- <br />mutely 1,400 acres. Annual precipitation over this area probably ranges from about <br />13 to 18 inches. Potential evapo-transpiration losses are greater than the annual pre- <br />cipitation. As a result, very little surface runoff ever occurs in Streeter Gulch and <br />then only following intense rainstorms or for a few days following a period of rapid <br />snowmelt in the spring. Taking into account the seasonal pattern of precipitation and <br />evapo-transpiration losses, and the elevation of the drainage area, it is estimated that <br />the average annual runoff is less than one inch of water depth, or no more than 100 <br />acre feet per year, from areas tributary to Streeter Gulch. Water balance studies made <br />by the U.S. Geological Survey yielded an estimated average annual runoff of 37 acre <br />~ feet for Streeter Gulch.~O~ <br />The foregoing values pertain to conditions which prevailed prior to mining. <br />During mining operations the intercepting of surface runoff from areas above the working <br />pit by contour drainage ditches as well as the greater porosity of the overburden spoil <br />will reduce both the volume and rate of surface runoff. After mining on the Lease has <br />been completed, the topography of the area will be such that the drainage area tributary <br />to the Gulch will be reduced to approximately 1, 150 acres with a corresponding reduc- <br />tion in annual runoff. <br />Hydrologic studies mode for the purpose of designing the relocated channel for <br />Streeter Gulch and the silt detention pond at its terminus resulted in estimates of flood <br />Table IV-9 of the Environmental Impact Assessment Report by VTN, December 1975. <br />4 <br />