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9.5 ~ 9.0 H Colorado water-quality standard Z ~ 8.5 0= a o a.o z a ~ 7.s z a 7.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 DATE SAMPLED 6.5 '--`- 1950 Figure 2. Temporal distribution of measured pH values at Yampa River near Maybell. to about 8.3 during the 1980's and 1990's. If the trend is real and were to continue to the point where pH frequently exceeded the Colorado water-quality stan- dard of 9.0, adverse effects on aquatic organisms, especially fish, would be expected. Of particular concern to the National Park Service and State and Federal fish and wildlife managers is the potential for adverse aquatic effects in Dinosaur National Monu- ment (fig. 1). The general objective of this study is to charac- terize trends in measured pH values and selected constituents in historical data and water-quality data collected for this study. Specific objectives of this study are to: 1. Characterize downstream trends in the water quality of the Yampa River at times of large pH values, 2. Estimate maximum potential pH in the lower Yampa River Basin due to enhanced photosyn- thesis and drought, 3. Evaluate available water-quality data for the Yampa River near Maybell for possible trends in measured pH and related constituents, 4. Determine if historical trends in measured pH values are real or the result of systematic bias in older pH data, and 5. Determine possible chemical causes of increasing measured pH values, if real. Purpose and Scope This report describes and tabulates synoptic and diurnal data collected for this study from selected sites along the Yampa River and main tributaries during August 1999 and March 2000. Geochemical interpre- tations of these data are presented. Synoptic samples are used to estimate the maximum potential diurnal pH due to photosynthesis in the lower Yampa River (including Dinosaur National Monument) for hypo- thetical scenarios of enhanced photosynthesis and drought. The historical water-quality data at the Yampa River near Maybell for the period 1950-99 are then interpreted statistically and geochemically to determine the nature of historically increasing, measured pH values. Finally, the report evaluates the validity of the apparent historical increase in measured pH values in the Yampa River. Description of Study Area The Yampa River Basin (fig. 1) is in north- western Colorado on the west slope of the Rocky Mountains. The headwaters of the river originate at altitudes exceeding 10,000 ft south of Steamboat Springs. The highest site sampled for this study, 09237500 Yampa River below Stagecoach Reservoir (site 1 in fig. 1), is at an altitude of about 7,200 ft. The mouth of the Yampa River (site 12 in fig. 1) at the Green River a few miles east of the Utah State line is at about 5,070 ft. The climate of the study area varies 4 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000