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The recharge, discharge and movement of ground water within the alluvial aquifers are controlled by: 1) the gradient of the rock channel into which the alluvium has been deposited. 2) the width of the alluvial deposit. 3) the thickness of the alluvial body, 4) the permeability of the alluvial deposit, 5) the recharge or discharge of underlying rock strata, and 6) the flow characteristics of the associated stream. The alluvial ground water aquifers are closely related to their associated streams. The alluvial aquifers receive a majority of their recharge from the streams and may sustain flows in the streams through their discharge. Recharge to bedrock aquifers occurs primarily in the spring and early summer when snow melt and surface water runoff is abundant, and when transpiration by vegetation and evaporation is not high, Recharge is limited by the permeability of rock strata and the limited recharge area exposed to surface water (e.g. most sandstones are cliff formers). Overlying clayey and silty soils, and shale and siltstone rock strata also limit vertical recharge of underlying sandstone and coal aquifers. Discharges from bedrock aquifers are primarily through springs to the surface system. The flow characteristics of the springs are related to: 1) the lateral extents and thicknesses of the associated aquifers; 2) the distance between the springs and the aquifer recharge areas; and, 31 the permeabilities and ground water storage characteristics of the associated aquifers. Springs issuing from laterally discontinuous, thin sandstones (lenticular and interbedded sandstones) which occur close to the aquifer outcrops will flow intermittently and their flows will fluctuate seasonally; while springs issuing from regionally extensive, thick sandstones or coals which occur at a distance from recharge areas will flow perennially and their flows will not significantly fluctuate with time. Surface Water The surface water general area included in the CHIS, consists of all surface drainage in the Upper Yampa River Subbasin above Hayden (Figure 11. The long-term estimated average annual flow from the Subbasin is about 1.1 million acre-feet (Steele, et al, 1979). Approximately 65 percent of this annual flow occurs during snow melt runoff in May and June (Steele, et al, 19741. Water resources development within the l)pper Yampa River Subbasin relies primarily on surface water supplies. The majority of municipal, industrial, domestic, stock and irrigation water supplies in the area are from surface water. Water quality (in terms of TDS concentration) in the Upper Yampa River Subbasin is variable and is related to both the geology of the source areas and climatic conditions. Surface waters draining high elevation snow pack areas are generally lower in dissolved solids due to the low soluble salt content of the igneous and metamorphic rock types in these areas. -22-