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nn2349 lakes in Colorado according to present or potential water uses. Major streams are separated into stream segments, and standards determined for each stream segment. The water-quality standards were derived to protect the classified uses of each segment; therefore, standards may be different for stream seg- ments for the same stream. The standard applied to a particular constituent could be based on U.S. Environmental Protection Agency drinking-water stand- ards (U.S. Environmental Protection Agency, 1986), standards for agricultural use, or standards based on protection of aquatic life. The water-use classi- fications and associated water-quality standards of selected constituents for stream segments of the Gunnison and the Uncompahgre Rivers within the study area and for Sweitzer Lake are listed in table 11. The State standards in table 11 for cadmium, copper, lead, mercury, and zinc are based on criteria for protection of aquatic life that were derived from acute or chronic toxi- city data (Colorado Department of Health, 1988). Except for the lower Uncompahgre River, the selenium standards are based on the drinking-water standard (10 ~g/L) or the standard for agricultural use (20 ~g/L). Table 11.--Water-u~e cla3sifications and water-quality standands {or selected constituents of the Colorado Department of Health, Water Quality Control Commission, for the Gunnison and Uncompahgre Rivers and {or Sweitzer Lake IClassification codes: RI, recreation with pdmary body contact, such as s....invning; R2, recreation, secondary contactj At agricultural, suitable for irrisation of crops and as drinking water for livestock; CWI, class 1 cold-water (water temperature seldom exceeds 20 degrees Celsius) aquatic life and able to sustain a wide variety of cold-water biota. including sensitive species; WWl, same as CWI only for warm-water aquatic life; WW2, class 2 warm-water aquatic life but nol capable of sustaining a wide vadety of warm-water biota. W2 standards protect only biota normally I jving in the wateri 0, domestic water supply; concentrations are in microsrams per liter, exc~pl for sulfate, chloride, and nitrate, which are iD milligrams per liter; --, standards for constituent not applicable for water-use class i ficat ions I St['ealll 'Section Chnifi- Sul- Chlo- Mi. A,rse- Boroo Cad- Ch['o- Cop- Lt'ad Her- St'le- or wate[' booy cations fate ride trate nit: mium mium nium Zinc pH cury Gunnilion River from Slllith Fork R2. A con f I u~nc~ to CWI, D 250 250 10 50 750 0.4 50 12 5 0.05 10 50 conflu~nce with Uncompahgre Rive[' Gunnison RiY~r fr~ confluence R2. A ~ith Uncompahgre CWI, D 250 250 10 50 750 3 100 20 33 .05 20 110 Riv~r to mouth Uncompahsre Riy~r from Ouray to R2, A 2 mi lea south CWI 50 750 .4 100 65 40 .05 20 225 of Hontro&~ Uncompahgre River feOIll 2 tIIi lea H2, A south of W2 50 750 5 100 30 50 .05 35 100 Montrose to the lDOutb Slo'~itzer Lake RI, A, WI 50 750 100 10 25 .05 20 50 37