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<br />-24- <br />Ground water flow in the Upper Yampa River Basin is controlled by the <br />geologic structure, stratigraphy and geomorphology of the general area. <br />The flows of ground water in the rock aquifers are controlled by the <br />structural folding and faulting and stratigraphy while the flows of <br />ground water in the alluvial aquifers are controlled by the geomorphology <br />of the stream valleys. <br />The Twentymile Park Structural Basin controls the regional ground water <br />movement within regionally extensive rock strata in the general area, the <br />Tow Creek sandstone, the Trout Creek sandstone, the Twentymile sandstone, <br />the coal seams, and the strata associated with the coal seams (Figure <br />3). The regionally extensive rock strata are recharged in the uplands <br />which rim Twentymile Park. The regional rock strata discharge downdip <br />either through the Hayden Syncline northwest to the Sand Wash Basin, or <br />to the surface streams in the Trout Creek system through outcrops and <br />subcrops in the northern and eastern edge of the basin, The regional <br />aquifers are under atmospheric pressure (water table conditions) near <br />their recharge areas and under hydrostatic pressure (artesian conditions) <br />within the basin or at discharge points. Artesian conditions are <br />developed in aquifers which are confined by overlying and underlying <br />strata with low permeabilities. Wells drilled into the regional aquifers <br />within the basin exhibit artesian flows. <br />The faults in the Twentymile Park Basin may provide a vehicle for ground <br />water communication between overlying and underlying aquifers and also <br />between aquifers and the overlying surface streams. The faulting <br />produces increased fracturing within and between aquifers (secondary <br />porocity). These more porous fault zones with their higher <br />permeabiTities can control both vertical and horizontal movement of <br />ground water between and within aquifers. In areas where hydrostatic <br />heads are lower in the underlying aquifers than the overly surface <br />elevation or the hydrostatic heads in overlying aquifers, the vertical <br />movement of water through the fault system would be either upward and <br />discharge to the surface system, or upward and recharge overlying <br />aquifers. <br />The lenticular and interbedded sandstones associated with the coal <br />bearing zones develop only localized discontinuous aquifers. Movement of <br />ground water within these sandstones is usually localized, with the flows <br />of water from the recharge to the discharge area occurring within a short <br />distance. Some limited regional movement of ground water may occur if <br />their is intercommunication between the sandstones. <br />The recharge, the discharge and the movement of ground water within the <br />alluvial aquifers are controlled by: 1) the gradient of the rock channel <br />into which the alluvium has been deposited, 2) the width of the alluvial <br />deposit, 3) the thickness of the alluvial body, 4) the permeability of <br />the alluvial deposit, 5) the recharge or discharge of underlying rock <br />strata, and 6) the flow characteristics of the associated stream. The <br />alluvial ground water aquifers are closely related to their associated <br />streams. The alluvial aquifers receive a majority of their recharge from <br />the stream and sustain baseflows in the streams through their discharge. <br />