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An examination of flow conditions during water year 1993 gives a general representation of water routing along the Colorado River, although flows during water year 1993 were above average. Major interbasin water transfers and reservoirs are shown in figure 9 and listed in table 3. The Colorado River below Baker Gulch represents flow in the headwaters, although the Grand River ditch upstream from Baker Gulch diverted an annual flow of 71 ft'/s in water year 1993. The Colorado River then flows into Shadow Mountain Reservoir/Lake Granby; from which water can be diverted through the Alva B. Adams Tunnel. This complex diversion and storage system was completed in 1950. Diversions from Grand Lake via the Alva B. Adams Tunnel to the South Platte River Basin are large during all months except June. The first major tributary to the Colorado River is the Blue River. The Harold D. Roberts Tunnel, operat- ing in conjunction with Dillon Reservoir, has diverted water from the Blue River Basin since 1963. Annual flows of 308 ft'/s (1993) for the Blue River are affected by Dillon and Green Mountain Reservoirs. The Eagle River is the next major tributary adding 542 ft;/s in water year 1993. Homestake Tunnel and Reservoir have diverted water from the Eagle River Basin since 1967. In water year 1993, the Colorado River had an annual flow of 2,330 ft'/s downstream from the Eagle River and 3,970 ft3/s downstream from the Roaring Fork, which had an annual flow of 1,540 ft'/s in water year 1993. The Twin Lakes Tunnel (completed in 1935) and the Charles H. Boustead Tunnel (completed in 1972) divert water from the Roaring Fork Basin. Ruedi Reservoir (completed in 1968) regulates the flow downstream from the diversions to the Charles H. Boustead Tunnel. Several small creeks flow into the Colorado River before it reaches Cameo where it had an annual flow of 4,667 ft'/s in water year 1993. The largest tributary to the Colorado River in Colorado, the Gunnison River, flows into the Colorado River at Grand Junction. Three small interbasin water transfers export water from the Gunnison River head- waters to the Arkansas and Rio Grande Basins. Three large dams were built for power generation and water storage on the main channel of the Gunnison River as part of the Colorado River Storage Project. Together the reservoirs constitute the Wayne N. Aspinall Unit, formerly known as the Curecanti Unit. The largest and farthest upstream reservoir is Blue Mesa Reservoir; the others are Morrow Point Reservoir and Crystal Reservoir. Large volumes of water are diverted within the basin through the Gunnison Tunnel, which transports water from the Gunnison River to the Uncompahgre Valley for irrigation (table 3). Streamflow in the Uncompahgre River at Delta is affected by inputs from the Gunnison Tunnel during April through October and by substantial evapotranspirationfiom about 90,000 acres of irrigated croplands. The annual flow for the Uncompahgre River in water year 1993 was 387 ft'/s. Ridgway Reservoir (completed in 1986) is located upstream from the Uncompahgre River at Delta station and affects the flow at this station. At the mouth of the Gunnison River near Grand Junction, the annual flow was 3,725 ft3/s in water year 1993. The natural flow of the Gunnison River is affected by diversions for irriga- tion ofabout 233,000 acres upstream from this station, storage reservoirs, and return flow from irrigated lands. The outflow of the basin, Colorado River near Colorado-Utah State line, had an annual flow in water year 1993 of 8,491 ft'1s, of which the Gunnison River contributed 44 percent. Ground Water In the study unit, ground-water resources have not been extensively developed. The most productive wells come from alluvial aquifers that are, formed from gravel, landslide, terrace, and glacial deposits in the basin. Other ground-water resources include consoli- dated aquifer systems and fractured systems such as those in metamorphic and granitic rocks. These aquifer systems generally yield less water than the unconsoli- dated aquifers. Some of the important aquifers in western Colorado in descending order of age are alluvial, Green River, Mesaverde, Mancos Shale unit, Dakota, Morrison, Entrada, Leadville, and Precambrian crystal- line unit (table 7) (U.S. Geological Survey, 1985). In some instances, alluvial aquifers might be hydrauli- cally connected to bedrock aquifers. Unconsolidated Aquifers Valley-fill alluvial aquifers along the Colorado River and other perennial streams provide some water for irrigation, public supply, and industrial use (U.S. Geological Survey, 1985). Alluvial aquifers in eroded intermontane valleys are thickest and most commonly saturated (Ackerman and Brooks, 1986). Thin aquifers are in alluvium and in eolian deposits on mesa tops. Aquifers on steep slopes of alluvium, talus, and colluvium are only seasonally saturated. The thickness of alluvial aquifers is less than the thickness of all bedrock aquifers in the basin. However, trans- missivity is much larger in the alluvial aquifers as compared to the bedrock aquifers. 24 Environmental Setting and Implications on Water duality, Upper Colorado River Basin, Colorado and Utah