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.... f~ ':) 20 .:..'J from about OOC in December through February to about 200C in July and August during 1986 through 1989. Mean daily specific conductance usually is largest from January to March when stream flows are smallest, and the smallest specific-conductance values occur during May through July when stream flows are the largest. During 1986 through 1989, the largest mean daily spe- cific conductance was 79311S/cm, which occurred dur- ing February 1989; the smallest mean daily specific conductance of 14711S/cm occurred in June 1986. Dissolved-solids concentrations vary proportionately with specific conductance. Calcium is the dominant cation and bicarbonate and sulfate are the codominant anions. Cain (1987) detennined relations between: (I) specific conductance and streamflow, (2) specific conductance and concentration of dissolved solids, and (3) specific conductance and concentrations of major ions. Hardness values at station 07097000 ranged from 66 mg/L (moderately hard) to 240 mg/L (very hard) as calcium carbonate. Hardness varies inversely with streamflow. Dissolved oxygen, pH, and alkalinity data sum- marized in table I indicate that the water at station 07097000 is: (I) well oxygenated, (2) alkaline (pH val- ues usually range from 8. I to 8.8), and (3) well buffered (median alkalinity equals 132 mg/L as calcium carbon- ate), Concentrations of total nitrogen at station 07097000 generally are less than 1.0 mg/L, and organic nitrogen is the dominant nitrogen species (table I). Edelmann (1988) reported that 61 percent of the total nitrogen consisted of organic nitrogen, 32 percent as nitrite plus nitrate, and 7 percent as ammonia. Addi- tionally, about 70 percent of the total nitrogen was dis- solved. Concentrations of total phosphorus ranged from 0.038 to 0.88 mg/L during 1986 through 1989; the median total phosphorus concentration was 0.08 mg/L. Based on the analyses of 34 water samples collected from 1986 through 1989, about 50 percent of the total phosphorus was dissolved. Concentrations of total and dissolved trace ele- ments analyzed from water samples collected at station 07097000 are summarized in table I, Trace ele- ments that have the largest concentrations are barium, iron, manganese, and zinc. Most of the samples col- lected were filtered and analyzed for dissolved concen- trations. However, several of the samples were analyzed for concentrations of dissolved and total trace elements. These analyses indicate that most of the trace elements are attached to suspended material; the largest concentrations of trace elements occur with the largest streamflows that transport relatively large quan- tities of suspended material. Methods of Investigation Following an initial reconnaissance of Pueblo Reservoir in May 1985, a comprehensive water-quality investigation of the reservoir was done in two phases. Phase I was done from the spring of 1985 through the fall of 1987 and included intensive collection ofreser- voir water-quality data from seven transects located from the inflow (transect I) to the dam (transect 7) (pI. 1). Water-quality samples were collected from seven transects in the reservoir in order to adequately represent any potential effects by tributaries or reser- voir morphometry. Three sampling sites were selected along each transect. Onsite measurements of water temperature, specific conductance, water transparency, dissolved oxygen, and pH were made at a1l21 sites, and water-quality samples were collected from the middle site in each transect. Onsite measurements were made monthly in the summer of 1985, biweekly in the sum- mer of 1986, and weekly in the summer of 1987. Water-quality samples generally were collected monthly in the spring, summer, and fall. A sample also was collected in the winter when weather conditions permitted, Phase II was done from the summer of 1988 through the fall of 1989. Analysis of data collected during Phase I indicated that data collected from sites 3B, 5C, and 7B adequately describe the spatial variations of the physical, chemical, and biological characteristics within the reservoir. Therefore, water- quality data collection during Phase 11 and most analy- sis and discussion of water-quality data for the study are limited to sites 3B, 5C, and 78. During Phase n, data were collected four times annually: (1) spring, (2) early summer, (3) late summer, and (4) fall. Biolog- ical data were collected twice in 1988 and four times in 1989. Bottom-sediment and interstitial pore-water samples were collected and analyzed as part of a sepa- rate, unrelated research effort (Callendar and others, 1988). Bottom-sediment samples were collected from 23 locations in October 1987 and analyzed for major chemical constituents. Additionally, samples of bot- tom sediments were collected for trace-element and nutrient analyses of the interstitial water in October 1987 from transects 2, 5, and 7 and in August 19887from transects 3 and 6. In September 1990, bottom- sediment samples were collected from transects 4 and 5 for nutrient analyses of the interstitial water, Water temperature, specific conductance, dis- solved oxygen, and pH generally were measured at 3-ft intervals from the reservoir surface to the reservoir bot- tom with a multiparameter instrument. Water transpar- ency was measured with a Secchi disk. Water samples 10 PhYllcal, Chemical, end Biological Characteristics of Pueblo Re.ervolr, Colorado, 1985-89