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carbon at site 6 in March 2000 compared to August 1999 probably resulted from flushing of the land surface by melting snow and ice along unsampled trib- utaries and the Yampa River valley between sites 5 and 6 during March 2000. This hypothesis is indicated by a 133-ft3/s increase in discharge between sites 5 and 6 during March 2000 compared to a 36-ft3/s increase during August 1999. Synoptic sampling during March 2000 showed a downstream distribution of pH (fig. 14) similar to the synoptic sampling during August 1999 (fig. 5). The largest difference was at Yampa River below Stage- coach Reservoir (site 1), where pH measured 7.77 in March 2000 (compared to 8.46 in August 1999). According to calculations using PHREEQC, this lower pH during March 2000 implies that COz was at 1,000 percent of saturation (compared to 300 percent during August 1999), probably because of winter stratification of Stagecoach Reservoir. Downstream from site 1, pH at Yampa River sites (fig. 14) averaged 8.85 during March 2000 (compared to 8.70 during August 1999), reflecting an average PCp (fig. 14) of 67 percent of saturation (compared to 99 percent during August 1999). Dissolved oxygen concentrations averaged 119 percent of saturation at Yampa River sites down- stream from site 1 during both sampling periods (figs. 5 and 14), but, because dissolved oxygen concentrations were about 40 percent larger at those sites during March 2000 (averaging 12.2 mg/L) than during August 1999 (averaging 8.6 mg/L), photosyn- thesis increased dissolved oxygen concentrations about 40 percent more during March 2000 than during August 1999. These relations indicate greater effect of photosynthesis on pH and dissolved oxygen concen- trations during March 2000 than during August 1999, despite shorter days and colder water temperatures in March 2000. However, this conclusion does not imply that rates of photosynthesis were greater during March 2000. Greater increase in pH and in dissolved oxygen concentrations by photosynthesis during March 2000 probably can be attributed to (1) slower rates of exchange of CO2 into and dissolved oxygen out of the river because of colder water temperature and deeper water and (2) slower rates of CO2 production and oxygen consumption resulting from slower rates of respiration by organisms and of aerobic decomposition of organic matter in the colder river water and streambed sediment. Calculations using PHREEQC indicate that synoptic samples collected at Yampa River sites during March 2000, if equilibrated with local, atmo- spheric PCp ,would have pH in the narrow range of 8.64 to 8.772(average 8.69)(fig. 15), compared to 8.55 to 8.80 (average 8.67) during August 1999 (fig. 6). These calculations indicate that the high alkalinity of Yampa River water causes high pH that is further elevated by CO2 removal during photosynthesis. Calculations using PHREEQC indicate that synoptic samples collected at Yampa River sites (except site 1) during March 2000 were oversaturated with calcite (fig. 14); the sample from site 1 was near saturation because of relatively small pH. Excluding site 1, the average saturation index was 0.98 during March 2000, compared to 0.84 during August 1999 (fig. 5), indicating greater oversaturation during March 2000-mostly a result of higher pH, alkalinity, and concentration of calcium. Additional calculations using PHREEQC indicate that synoptic samples collected at Yampa River sites during March 2000, if equilibrated with local, atmospheric PCO2 and calcite, would have pH in the narrow range of 8.43 to 8.50 (average 8.45) (fig. 15), compared to 8.42 to 8.50 (average 8.45) during August 1.999 (fig. 6). The general agreement of simulated pH values for the two sampling periods indicates that pH in the Yampa River was largely controlled by degree of PCp saturation and degree of oversaturation with calcite during both sampling periods. Estimate of Maximum Potential Late-Afternoon pH in the Lower Yampa River Basin Enhanced photosynthesis and increased alka- linity in the lower Yampa River Basin have the poten- tial to increase maximum diurnal (late-afternoon) pH. To estimate maximum potential pH at water-quality sites in the lower Yampa River Basin, water-quality samples collected at Yampa River above Elk River (site 3, fig. 1) on August 18, 1999, and March 13, 2000, were selected to represent hypothetical, poten- tial conditions of photosynthesis in the lower basin. PHREEQC was used to calculate the difference in the concentration of CO2 gas in the synoptic samples from site 3 at measured pH and the concentration in the samples at equilibrium with ambient atmospheric PCp2. These calculations indicate that, relative to CO2 concentrations in equilibrium with the atmosphere at 20 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000