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rivers in North America to be 68 mg/1. By this com- parison, it might be said that the reach of Arkansas River known as Pueblo Reservoir is an "average" car- bonate river, Alkalinity greater than 400 mg/I for pub- lic water supplies is usually considered undesirable. Pueblo Reservoir water never approached such a level. For aquatic wildlife and freshwater organisms no maxi- mum has been established, but 20 mg/I is the minimum. Again, the reservoir waters are quite acceptable regard- ing such a minimum. Chloride. - Chloride is the third most common anion in temperate waters. On the basis of mill i-equivalents (Cole [12] and Wetzel [15]). The seasonal concentra- tions of chloride in the inlet, outlet, and pool are shown in figure 9. All three were usually closely re- lated from month to month. The annual mean chloride levels for the pool for 1974 and 1975 were 9.8 and 9.3 mg/l, respectively, The highest and lowest monthly averages for the pool for 1974 and 1975 were 12.5 (March) and 12,5 (February) and 5.3 (July) and 4.6 (July), respectively. Wetzel [15] cites the mean chlo- ride concentration of North American rivers as 8 mg/1. Again, the mean concentration in Pueblo Reservoir water closely approximates the "average" North Amer- ican river. Pre-impoundment chloride concentrations (appendix C) were more extreme and variable than those during the 1974-76 impoundment period. Chlo- ride in water for use as a public water supply should not exceed a desirable maximum of 250 mg/1. Irrigation water should not exceed 700 mg/l chloride. In neither application is there evidence that water of Pueblo Reservoir exceeds such maxima. Sulfate. - Annual mean sulfate concentrations for the reservoir pool for 1974 and 1975 were 152 and 121 mg/I, respectively, The highest and lowest monthly averages were 232 (March) and 175 (March) and 51 (June) and 56 (June), respectively, In both 1974 and 1975, Mafch was highest and June lowest (fig. 11). During pre-impoundment river conditions (fig. 23), highest sulfate concentrations were usually higher than pool highs and occurred in either December or January. Lowest sulfate concentrations during pre- and post- impoundment periods were in June and were much the same. A sulfate concentration of < 50 mg/I is a desir- able level as a criterion for water use as a public water supply. A permissible maximum of 250 mg/l has also been established [11]. Pool water of Pueblo Reservoir rarely satisfied the former criterion level, but never exceeded the latter. Total hardness. - Hardness in natural waters is chiefly due to dissolved calcium ~nd magnesium ions. The sea- sonal changes in total hardness as mg/I CaC03 for the reservoir inlet, outlet, and pool are shown in figure 10. The mean annual averages f total hardness for 1974 and 1975 were 215 and 23 , respectively. The highest monthly means for each y ar were 283 (March) and 439 (December), respectivel . The lowest monthly aver- ages for each year both occ rred in June and were 106 and 102, respectively. Hard ess of more than 300 to 500 mg/l as CaC03 is exce sive for public water sup- plies. Only rarely did water f Pueblo Reservoir exceed the 300 level maximum. Th term hardness should be avoided in reference to wat r quality requirements for aquatic life. Calcium hardness. - Calciu hardness as mg/I CaC03 and the equivalent concent ation as calcium alone are listed below for the appro riate pool means of the dissolved fraction only: 1974 Annual Highest Lowest Mean Monthly Monthly mg/I CaC03 149 185 74 (Dec., Mar.) (June) mg/I Ca 60 74 30 1975 Annual Highest Lowest Mean Monthly Monthly mg/I CaC03 167 329 78 (Dec.) (June) mg/I Ca 67 132 31 The calcium content (mg/l) in the suspended fraction for each of the aforementio ed categories in the same order were as follows: 1974 mg/l Ca 0.66 1.65 0.1 (Apr.) (Oct., ov.) 1975 0.50 1.49 0.28 (July) (Mar.) The dissolved calcium lev Is were about 100 times greater than the suspended calcium levels. Dissolved calcium content was usual y greatest during highest discharge (June). The seaso al trends of calcium hard- ness for the reservoir pool fo the 1974-76 study period followed that of pre-impou dment river conditions of 1972-74 (fig. 23). No spe ific water quality criteria have been established for d ssolved or suspended cal- cium concentrations. From table 4 it is apparent that little, if any, calcium enric ,ment or accumulation has occurred in the inlet sedime ts. Magnesium hardness. - Ma nesium hardness as mg/l CaC03 was measured in th dissolved fraction water from the inlet, outlet, and 001 of Pueblo Reservoir, 24