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Surface Water <br />The area to be mined is small compared to the area of the drain- <br />ages which include the site. Tabel 24 provides estimates of the <br />volumes of water contributed by the proposed mined area to <br />Hayden Gulch and Dowden Gulch under natural conditions. Dry <br />Creek is not listed, since the portions of the Dry Creek water- <br />shed within the permit area do no include any land to be mined. <br />Surface water yield will be impacted in four main ways by active <br />mining <br />• Slight increase in yield due to an increase in the <br />percentage of runoff from portions of the area being <br />mined. <br />• Slight increase in yield due to ground water inflow to <br />the mine. <br />• Moderate decrease in yield due to modified topo- <br />• graphy restricting runoff, evaporation from sediment <br />ponds, and evaporative loss of water taken from ponds <br />for dust suppression. <br />• The timing of the runoff from the site will be modified <br />by the use of detention ponds to control the sediment <br />content of the runoff. <br />The net reduction in surface water yield to Hayden Gulch during <br />the period of active mining may be on the order of three acre <br />feet per year. This would decre ase the flow at gage S-4 b y one <br />percent. This reduction would be negligible when compared to <br />natural year-to-year fluctuations in flow. Dowden Gulch will be <br />impacted, to a less er degree, during the last few years of <br />mining . <br />Surface runoff to Hayden and Dowden Gulch after mining should <br />approximate the pre-mining runoff because: <br />. , • Post-mining topography will approximate pre-mining <br />topography <br />