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,-, co ~ <0 :~J ,~ --' r Chapter I II AFFECTED ENVIRONMENT AND ENVIRONMENTAL CONSEQUENCES Table 4 Average TDS concentrations at major river stations (U.S. Department of the Interior, 1989) River stations maiL Colorado River near Cameo, Colorado Gunnison River near Grand Junction, Colorado Colorado River near Cisco, Utah 383 445 496 On the Colorado River, between the Cameo gauge and the Colorado River above the Dolores River, the Plateau Creek adds flows with relatively low salinity levels, while the Gunnison River adds flows with higher salinity levels. Other ungauged sources of saline water within the reach of the river include intermittent side drainages. The total salt loading contribution from ungauged sources is estimated at 16,000 tons annually (Reclamation, 1986b). Sediment yields from erosion in the Grand Valley are generally moderate south of the canal systems. The yield is approximately 0.2 to 0.5 acre- foot per square mile per year in this area, including that from cropland. In the area north of the canals, sediment yields are extremely high, ranging from approximately 1.0 acre-foot to 3.0 acre-feet per square mile per year in this area, which includes lands with little vegetative cover and numerous deep gullies (Colorado Land Use Commission, 1974). In the Badger Wash area, sediment yields were measured and ranged from 7.4 to 30.0 acre-feet per square mile for the 5-year period--1954 through 1958 (Geological Survey, 1963). Ground Water.--The quality of ground water in the Grand Valley Unit area varies with depth, thickness of the aquifer layers, proximity to recharge area, and the time of year. Ground water from the weathered portion of the Mancos Shale aquifer discharges directly into many drains in the Unit area. Outflow drains are almost entirely supplied by the ground-water system in the winter, and the poor quality of winter flows in drains represents typical ground-water quality in the area. The salinity in the washes and drains are much greater than those in canals and laterals, primarily because of the poor quality of the ground-water inflow. The annual flow-weighted average salinity content of the drains and washes is about 1,380 mg/L and ranges from 970 to 2,600 mg/L. Concentrations vary with the season; they are lowest during the irrigation season, when administrative wastewater and surface return flows dilute flows in drains, and highest during the non-irrigation season when flows derived from ground-water sources are undiluted. More than 95 percent of the salt loading from the Unit area is from ground- water sources, most of which is attributable to agricultural practices. Table 5 shows the estimated amount of salt loading from each ground-water recharge source. 20