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Ground water flow in the Upper Yampa River Basin is controlled <br />by the geologic structure, stratigraphy and geomorphology of <br />the general area. The flows of ground water in the rock <br />aquifers are controlled by the structural folding and faulting <br />and stratigraphy, while the flows of ground water in the <br />alluvial aquifers are controlled by the geomorphology of the <br />stream valleys. <br />The Twentymile Park Structural Basin controls the regional <br />ground water movement within regionally extensive rock strata <br />in the general area, the Tow Creek Sandstone, the Trout Creek <br />Sandstone, the Twentymile Sandstone, the coal seams, and the <br />strata associated with the coal seams (Figure 3). The <br />regionally extensive rock strata are recharged in the uplands <br />which rim Twentymile Park. The regional rock strata discharge <br />downdip either through the Hayden Syncline northwest to the <br />Sand Wash Basin, or to the surface streams in the Trout Creek <br />system through outcrops and subcrops in the northern and <br />eastern edge of the basin. The regional aquifers are under <br />atmospheric pressure (water table conditions) near their <br />recharge areas, and under hydrostatic pressure (artesian <br />conditions) within the basin or at discharge points. Artesian <br />conditions are developed in aquifers which are confined by <br />overlying and underlying strata with low permeabilities. Wells <br />drilled.lnto the regional aquifers within the basin exhibit <br />artesian flows. <br />Since faulting produces increased fracturing within and between <br />aquifers (secondary Dorosityl, the faults in the Twentymile <br />Park Basin may provide a vehicle for ground water communication <br />between overlying and underlying aquifers and also between <br />aquifers and the overlying surface streams. In areas where <br />hydrostatic heads are higher in the underlying aquifers than <br />the overlying surface elevation or the hydrostatic heads in <br />overlying aquifers, the vertical movement of water through the <br />fault system would be upward and either discharge at the <br />surface or recharge overlying aquifers. <br />The lenticular and interbedded sandstones associated with the <br />coal bearing zones generally develop localized discontinuous <br />aquifers. Movement of ground water within these sandstones <br />from the recharge to the discharge area usually occurs within a <br />short distance. Limited regional movement of ground water <br />occurs when there is intercommunication between the sandstones. <br />_ 21 _ <br />