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Diurnal measurements at Yampa River near Maybell (site 8 in fig. 1) during August 23-24, 1999 (fig. 9), showed smaller effects of photosynthesis and respiration plus oxidation of organic matter than at sites 3 and 6. Values for pH peaked at 8.66 at 1900 hours on August 23 and attained a minimum of 8.52 at 0700 hours the next day (a small diurnal change of 0.14 unit). Dissolved oxygen peaked at 132 percent saturation at 1500 hours on August 23 and attained a minimum of 86 percent saturation at 0100 hours and 0600 hours the next day. Specific conductance ranged from 474 to 502 µS/cm. Diurnal measurements at Yampa River at Deer- lodge Park (site 11 in fig. 1) during August 25-26, 1999 (fig. 10), showed little or no effect of photosyn- thesis on pH and dissolved oxygen saturation. The smallest pH values (8.45-8.51) were measured during daylight on August 25, and the largest pH values (8.59-8.60) were measured between the hours of 2200 on August 25 and 0500 hours on August 26 (an inverted diurnal change of 0.15 unit). Dissolved oxygen concentration peaked at 106 percent of satura- tion at 1700 hours on August 25 and reached a minimum at 90 percent of saturation at 0700 hours the next day, with concentrations varying in the narrow range from 6.7 to 7.3 mg/L and remaining within 5 percent of saturation for most of the measuring period. Specific conductance ranged from 555 to 571 µS/cm. Cloudy weather to about 1330 hours and from about 1630 hours until after sunset on August 25 probably suppressed photosynthesis, the presence of which was weakly evidenced by the pattern of dis- solved oxygen saturation. However, cloudy weather during daylight cannot account for the larger pH during night, which indicates a change in the chem- istry of the water flowing past site 11. Diurnal measurements at Yampa River at mouth (Echo Park) (site 1.2 in fig. 1) during August 25-26, 1999 (fig. 11), showed substantial effects of photosyn- thesis on dissolved oxygen concentration but not pH. Values of pH varied slightly from 8.60 to 8.70, with the largest values generally occurring between 2200 and 0200 hours. Dissolved oxygen concentrations ranged from 6.8 to 8.4 mg/L, peaking at 118 percent of saturation at 1500 hours on August 25, which indi- cated that photosynthesis was occurring, and attained a minimum of 93 percent of saturation at 0700 hours the next day, which indicates slight respiration plus oxida- tion of organic matter. Specific conductance ranged from 581 to 591 µS/cm. Except for a brief sunny period during midafternoon, cloudy weather on August 25 probably suppressed photosynthesis. As at site l 1, larger pH during night indicates a change in the chemistry of the water flowing past site 12. Diurnal measurements on the Yampa River during August 23-26, 1999, showed a general down- stream decrease in the effects of photosynthesis and respiration plus oxidation of organic matter. From Yampa River above Elk River (site 3) to Yampa River below Craig (site 6) to Yampa River near Maybell (site 8), there was a decrease in maximum daily pH values (9.07 to 8.78 to 8.66), in diurnal fluctuation in pH values (1.15 to 0.61 to 0.14 units), and in maximum daily dissolved oxygen saturation (178 to 140 to 132 percent); these trends show that eutrophica- tion of the river and the effects of photosynthesis decreased substantially upstream from site 8. In addi- tion, the downstream increase in minimum dissolved oxygen saturation (66 to 78 to 86 percent) indicates that respiration plus oxidation of organic matter decreased substantially between sites 3 and 8. The small diurnal fluctuation in pH values (0.14 unit) at site 8 indicates that respiration plus oxidation of organic matter during afternoon hours produced CO2 at about the same rate as it was consumed by photo- synthesis; this interpretation is supported by the 89 percent of saturation with CO2 at site 8 during synoptic sampling on August 16, 1999 (fig. 5). During night, when photosynthesis does not occur, respiration plus oxidation of organic matter, although slower because of cooler temperature, would have continued to produce CO2 and lower pH. Cloudy weather during most of the day when diurnal measurements were made at sites 11 and 12 made interpretation of those measurements more prob- lematic. However, larger pH values at those sites during night (figs. 10 and 11) indicates that (1) source- water chemistry varied with respect to capacity for respiration plus oxidation of organic matter and photo- synthesis and (2) photosynthesis was minor and pH was largely controlled by relative rates of CO2 produc- tion caused by respiration plus oxidation of reactive organic matter, which would have been greater during warmer daylight hours, lowering pH. The second point is supported by oversaturation of CO2 in river water at sites 11 and 12 during synoptic sampling (fig. 5). However, this observation does not discount the possi- bility that variation in source-water chemistry also affected diurnal pH. 14 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000