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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 the flows of water from the recharge to the discharge area occurring within a short distance. Some limited regional movement of ground water may occur if there is intercommunication between the sandstones. The recharge, discharge and movement of ground water within the alluvial aquifers are controlled by: 11 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 stream and sustain baseflows in the streams through their discharge. Recharge to aquifers occurs primarily in the spring and early summer when snowmelt and surface water runoff is abundant, and when transpiration by vegetation and evaporation is at a minimum. Recharge to rock aquifers 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 strata 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 3) 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 flow intermittently and their flows fluctuate seasonally; while springs issuing from regionally extensive, thick sandstones or coals which occur at a great distance from recharge areas flow perennially, and their flows do not significantly fluctuate with time. a. Surface Water The following description of surface water in the Yampa River Basin discusses the general physical characteristics of the drainage, detailed narrative of flow regimes and an analysis of water quality parameters. Flow and water quality characteristics of natural springs and spoils springs are noted. Water use in the basin is also described. The major perennial tributaries to the Yampa River are, from upstream to downstream, Oak Creek, Elk Creek, Trout Creek, Grassy Creek, Elkhead Creek, and the Williams Fork. Trout Creek, Grassy Creek, the Williams Fork and Dry Creek an intermittent tributary, are affected by mining. Trout Creek has two major tributaries which have mining-related impacts: -37-