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partial pressure of carbon dioxide gas (PCO2) averaging 67 percent of saturation (compared to 99 percent during August 1999). The apparently larger effects of photosynthesis on pH and dissolved oxygen concentrations during March 2000 compared to August 1999 probably were caused by (1) slower rates of exchange of C02 into and dissolved oxygen out of the river because of colder and deeper water and (2) slower rates of C02 production and oxygen consumption resulting from slower rates of respiration by organisms and from slower rates of aerobic decomposition of organic matter in the colder river water and streambed sediment. Hypothetical thermodynamic simulations were done for samples collected in the lower Yampa River Basin to simulate the same amount of photosynthesis that existed at Yampa River above Elk River. These simulations indicate that maximum potential late-afternoon pH would equal about 9.1 to 9.2 (exceeding the Colorado water-quality standard of 9.0) during late-winter lowland runoff and during late-summer base flow. Additional simulations indicate that late-summer drought conditions could further raise maximum potential late-afternoon pH by about 0.1 unit, potentially causing late-afternoon pH to remain above the water-quality standard. Flovr~-adjusted, two-tailed Wilcoxon-Mann- Whitney rank-sum tests were used to compare onsite measurements, constituent concentrations, and thermodynamic properties of water samples collected from Yampa River near Maybell between 1950-74 and 1975-99. These two periods were defined to represent the general periods of time before and after onsite measure- ments of pH were begun and to separate the earlier period of minor coal-mining activity from the period of more extensive coal mining that began in the late 1970's. Specific conductance, concentration of dissolved solids, dissolved-solids load, measured pH, and dissolved concentrations of calcium, magnesium, sodium, and sulfate were significantly greater during 1975-99. Dissolved concentrations of chloride, fluoride, and silica were significantly greater during 1950-74. Alka- linity and dissolved potassium concentration were not significantly greater during either period. The C02 saturation factor was signifi- cantly greater during 1950-74 (median 10.2) than during 1975-99 (median 2.5). However, hypothet- ical equilibration of all samples with ambient atmospheric pressure of C02 resulted in no signif- icant difference in pH for the two periods. There- fore, the significantly greater measured pH values during 1975-99 cannot be attributed to the signifi- cant increase in concentrations of dissolved solids, calcium, magnesium, sodium, and sulfate, leaving decrease in the partial pressure of C02 as the most likely cause. Greater dominance of rates of respiration plus oxidation of organic matter (relative to rates of photosynthesis) during 1950-74 and (or) greater dominance of rates of photosynthesis (relative to rates of respiration plus oxidation of organic matter) during 1975-99 possibly contrib- uted to the significantly smaller measured pH values at Yampa River near Maybell during 1950-74, although these causes were not the primary cause. Most of the significant difference in measured pH between the two periods can be attributed to oxidation of organic matter in sample containers during shipping and holding prior to laboratory measurement, especially in composited samples analyzed before 1970. Laboratory measurements and underestimated onsite measurements before September 6, 1983, prob- ably are less reliable than onsite measurements made after that date. INTRODUCTION In 1999, the U.S. Geological Survey (USGS) began a study of pH trends in the Yampa River from Stagecoach Reservoir near Steamboat Springs, Colo- rado, to its mouth, a few miles from the Utah State line (fig. 1). The study was initiated because of an apparent historical increase in measured pH values (fig. 2) at the USGS stream gage 09251000, Yampa River near Maybell, Colorado (fig. 1), since the 1960's. The data indicate that, on average, measured pH values increased from about 7.6 during the 1950's and 1960's 2 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000