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4.4 Reservoirs and Dams The jl!stification for the Segment 7 classification is that: The Woodling Study indicates that Mineral' Creek, from its source to its confluence with South Mineral Creek, is highly toxic due to inineralization and there is not a likelihood that the sources of that toxicity will be corrected in 20 years. However, the Commission concluded that there was likely to be aquatic life in that portion of Mineral Creek from below South Fork to Silverton. All three sections have been severely impacted by anthropogenic pollution from mineral development (Harvey, personal communication), yet Colorado has explicitly chosen to maintain the status quo by prescribing the less stringent standards of a Recreation f classification. , In evaluating its surface waters for its 305(b) Report, Colorado has modified the EPA's suggested use support classifications (Tables 16a and 16b) (Colorado Water Quality Control Division 1992). According to the 1992 report, portions of all major tributaries to the San Juan River in Colorado fail to fully support their uses (Table 17). Narraguinnep, McPhee, and Navajo reservoirs were each found to partially support their uses, all due to mercury levels in fish. Of the nineteen river or stream reaches whose uses were impaired, metals were cited as contaminants in twelve, sediment in eight, and salinity in three. Sources of nonsupport are not listed., It should be noted that several stream reaches were ~esignated as Water Quality Limited (wQL), a category in which uses are not measurably impaired but for which there are indications that the potential exists for impairment in the near future (Colorado Water Quality Control Division 1992). Utah - Utah has a total of 12 water use classifications (Table 18). All river and stream segments within the San Juan basin are designated as fisheries, as is Lake Powell. The use classifications each have been assigned standards (Table 19). For the aquatic wildlife classifications, there are both acute (1 hour) and chronic (4-day) classifications. Criteria for domestic, recreation, and agricultural uses (Table 20), aquatic wildlife (Table 21), and the protection of human health (Table 22) are listed separately (Utah Department of Environmental Quality '1992). For Utah's 1991 accounting of water qualijy, one or more violations of acute or chronic toxicity criteria within a three-year period resulted in a determination of nonsupport for a stream classified for aquatic use life support. Use support was determined for stream segments within waterbodies that were monitored (sampled at least quarterly), assessed (sampled less than quarterly), or evaluated Qudgements were made whether similar waterbodies within a watershed had the same use support as those monitored or assessed) (Toole 1992). ' Within the San Juan basin, the Utah DivisionjofWater Quality sampled two stream segments from October 1, 1988 to September 30, 1991. Water from Montezuma Creek was sampled at a point 1.5 l1).iles upstream from the town of Montezwna Cre~k; at this station, the creek failed to support its fishery use as a result of temperature, dissolved oll.)'gen, copper, and iron exceedances. Water was also sampled from the San Juan River above Aneth; at this station, the mean iron concentration exceeded the fishery criteria of 1.0000 mgll, and copper and zinc ,standards were exceeded in 14.3% and 28.6% samples, respectively (Utah Division of Water Quality 1993). 4.4 RESERVOIRS AND DAMS . Dams and their reservoirs can significantly :alter downstream water quality. The following disclission lists a number of effects that may result from the construction and operation of dams. It should be noted that conditions favoring one or more 'effects might exclude others (Yahnke, personal communication). As a result of metabolism or other mechanisms; reservoirs can remove nutrients such as nitrogen and phosphorus from the water, reducing their concentrations below dams and effectively lowering the 62