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The ~.vidth of the alluvial ~+e?osit; The thickness of the alluvial body; 4. The permeability of the alluvial deposit; The recharge to or discharge from the underlying rock strata; and, The flow characteristics of the associated stream. The alluvial aquifers are closely related to their associated streams. The alluvial aquifers receive a majority of their recharge from the stream during large spring flows. Discharge from the alluvial aquifers provides baseflow to the streams. Ground water flow in the upper Yampa River Basin is controlled by the geologic structure, stratigraphy and geomorphology of the area. Ground water flow in the regional bedrock aquifers is controlled by the structural deformation and depositional history of the strata. The flow of ground water in the alluvial aquifers is controlled by the geomorphology of the stream, the valley's surface water flow volume and volume and composition of the alluvium. The structure of the Twentymile Park Basin controls the regional ground water flow in the regionally extensive water-bearing rock strata of the basin. The strata are recharged ;;i t::e uplands which rim the Twentymile Park Basin where the aquifers crop out. The ground water flows downdip discharging southwest through the Hayden Syncline to the Sand Wash Basin and northeast to Trout Creek and its tributaries that pass through outcrops of the aquifers. The regional aquifers are unconfined and partiaily saturated near their recharge areas. These units are confined, artesian aquifers in the basin and discharge zones. We11s drilled into the regional aquifers in the central part of the basin exhibit artesian flow. Vertical ground water flow may occur between bedrock aquifers. The potentiometric head in aquifers in the basin increases with depth creating a vertical head gradient. This vertical gradient may cause vertical ground water flows. Flow through low permeability confining layers will be very small, Significant flows may occur where the sedimentary strata have been offset by faulting. Fault zones may have increased permeability, (secondary permeability) due to fracturing of the strata. The faults in the Twentymile Park Basin may provide a path for ground water flow between separate aquifers and between bedrock aquifers and the surface streams. Faulting may produce increased fracturing (secondary per-neabitity) along the fault and, therefore, increased hydraulic conductivities. These fault zones with potentially greater hydraulic conductivities control both vertical and horizontal movement of ground water between and within aquifers. The lenticular and interbedded sandstones associated with the coal-bearing zones develop only localized discontinuous aquifers, movement of ground water within these sandstones is usually localized, with recharge and discharge areas occurring in close proximity to each other. Some limited regional movement of ground water may occur if there is interconnection between the sandstones. 33