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Section 3 Physical Environment of the Major River Basins gage. As the table indicates, an average of about 4.5 million AF of water leaves the state annually via the Colorado River at the state line. 3.3.7 Colorado Basin Groundwater Most of the water used within the Colorado Basin comes from surface water sources. Annual groundwater withdrawal data from 1995 indicate groundwater use by the counties encompassing the river basin varies from less than 1 percent in Grand and Mesa Counties to a maximum of 9 percent in Summit County (Solley et al. 1998). Because of the shallow well depths and water levels, alluvial groundwater is readily developed in rural areas for agricultural and domestic purposes. Aquifers located within the Colorado Basin are as follows (CGS 2003): ^ Alluvial Aquifer ^ Piceance Basin ^ Eagle Basin ^ Dakota-Cheyenne ^ Middle Park Basin Figure 3-13 presents the aquifers broken down into two groups: alluvial and bedrock (Piceance Basin, Eagle Basin, Dakota-Cheyenne, and Middle Park Basin). The distribution of alluvial deposits in the Colorado Basin varies greatly from one reach to the next. The alluvial deposits, as mapped by USGS geologic quadrangle maps, are primarily located near the Towns of Eagle and Gypsum, along the Roaring Fork River, Roan Creek, and from the Town of Palisade to the Colorado-Utah state line. Alluvium is very limited or non-existent in the canyon sections of the Colorado River where the bedrock is exposed (CGS 2003). The saturated thickness of the alluvium in the basin is represented by the interval from the water table to the underlying bedrock. Welder (1987) reported that test holes in the alluvium of Roan and Parachute Creeks penetrated 80 feet and 70 feet, respectively, of saturated permeable sand and gravel. For the Fraser River, Apodaca and Bails (1999) report alluvial saturated thickness ranging from 14 to 45 feet, averaging 21 feet in the spring, and ranging from 7 to 20 feet in the fall with an average of 15 feet. Private wells used for domestic and agricultural irrigation uses are common throughout the watershed (Colorado Groundwater Association 1999). Major production wells (those with rights that exceed 500 gpm) are also shown in Figure 3-13. 3.3.8 Colorado Basin Water Quality Upper Colorado River watershed water quality issues largely are related to impacts due to growth, mining, and the protection of threatened and endangered fish species. Growth related water quality issues are becoming increasingly important as the population continues to grow at rates among the highest in Colorado. Sediment and nutrient loading to streams in the watershed have the potential to create significant water quality problems. These loadings are caused primarily by runoff from construction activities at new subdivisions, commercial centers, roads, ski area expansions, and naturally erosive soils (CDPHE 2002). Salinity has long been recognized as one of the major issues on the Colorado River. The salt loads in the river system originate primarily from easily eroded saline-rich sedimentary rocks that are extensive in the lower basin. The Colorado River Basin Salinity Control Program is designed to prevent a portion of this salt supply from moving into the river system (Colorado River Basin Salinity Control Forum 2002). ~ - . • - ~ . ~ ~ . ..- . ~ I .. I I • I I Blue River below Green Mountain 09057500 328,785 454 1942-2002 599 Reservoir Eagle River below Gypsum 09070000 412,586 570 1946-2002 944 Roaring Fork at Glenwood Springs 09085000 877,836 1,213 1906-2002 1,451 Plateau Creek near Cameo 09105000 128,999 178 1936-2002 592 Colorado River near Kremmling 09058000 733,654 1,013 1962-2002 2,382 Colorado River near State Line 09163500 4,555,526 6,292 1913-2002 17,843 Source: USGS NWIS web/HydroBase database C~A ~J~~a Siatew~ide Water Supply Inii'iative 3-14 S:\REPORT\WORD PROCESSING\REPORT\S3 11-8-04-WITH PAGE BREAKS.DOC