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, 4,3.2 State Surface Water Quality Standards waters of the San Juan basin in New Mexico has fully supported uses. Agriculture and resource extraction activities are the most common sources of nonsupport, with metals and siltation as the most common causes of nonsupport (New Mexico Water Quality Control Commission 1992). The only San Juan basin lake whose uses are not fully supported is Navajo Reservoir (Table 8). Colorado - Colorado has eight surface water use classifications, one of which is for wetlands (Table 9) (Colorado Water Quality Control Conunission 1993a). All river and stream segments within the San Juan basin are designated fisheries with the exception of single segments along the Animas River, Cement Creek, and Mineral Creek. Individual river basins in Colorado have their own surface water quality standards. The standards for the San Juan basin and the Dolores River basin are grouped together (Table 10) (Colorado Water Quality Control Commission 1993b). The significance of the Dolores River to the San Juan basin is discussed in Section 4.10.3. Standards pertaining to the entire state for organics (Table 11), physical and biological parameters (Table 12), inorganics (Table 13), and metals (Table 14) have also been promulgated (Colorado Water Quality Control Conunission 1993a). All waters in the San Juan basin are subject to the following temperature standard: temperature shall maintain a normal pattern of diurnal and seasonal fluctuations with no abrupt changes and shall have no increase in temperature of magnitude, rate, and duration deemed deleterious to resident aquatic life (Colorado Water Quality Control Commission (1 993b ). 1n addition to the general standards set for the state and the San Juan basin, most segments within the basin have also been assigned standards specific to their designated uses. The segments, their classifications, and corresponding standards are listed by subbasin (Table 15) (Colorado Water Quality Control Commission 1993b). The Colorado Water Quality Control Commission (1993b) warns that although none of the water quality standards are set below detectable limits, routine methodology may not achieve a low enough detection limit for certain parameters. This warning l\pplies to several of the New Mexico and Utah standards as well, particularly for the more toxic parameters such as mercury and many of the organics. The classifications for upstream segments of streams generally are the same or higher than downstream segments. In a few cases, tributaries have been assigned lower classifications than mainstems where flow from the tributaries does not threaten the mainstem water and the lower classification is appropriate (Colorado Water Quality Control Commission 1993b). The three segments that have not been designated as fisheries are Segments 2, 6, and 7 of the Animas River basin (Table 15). The justification for the Segment 2 classification is that: Although there is some evidence of insect life ,at points in the segment, the evidence regarding the presence of aquatic life is contradictory, and there is no evidence of fish life being present. In the absence of sufficient data to support the classification of any portion of this segment for aquatic life, the current status is being retained and no aquatic life use is assigned. The justification for the Segment 6 classification is that: Since Cement Creek and its tributaries are degraded by abandoned mine drainage and past discharges, the Commission did not assign aquatic and agricultural classifications to the segments as had been proposed. The segment does not currently have an aquatic life classification, and thus the status quo is maintained. 32