Water Resources of Colorado

Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950–2000

by Daniel T. Chafin

Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 02-4038, 41 p., 25 figs.

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In 1999, the U.S. Geological Survey began a study of pH trends in the Yampa River from near its headwaters to its mouth. The study was prompted by an apparent historical increase in measured pH at the Yampa River near Maybell, from an average of about 7.6 in the 1950's and 1960's to about 8.3 in the 1980's and 1990's. If real, further increase could cause more frequent exceedances of the Colorado water-quality standard of 9.0 and adversely affect aquatic life in the Yampa River Basin, including Dinosaur National Monument. The principal conclusion of this study is that this apparent historical increase in measured pH was caused mostly by changes in measurement protocol.

Synoptic sampling during August 1619, 1999, a period of relatively warm weather and base flow, showed that late afternoon pH of the Yampa River ranged from 8.46 to 9.20. The largest pH (9.20) exceeded the Colorado water-quality standard and was measured at Yampa River above Elk River, about 1.8 miles downstream from the Steamboat Springs Regional Waste Water Treatment Plant outfall, where nutrient enrichment caused photosynthesis by algae to dominate. Here, the dissolved oxygen concentration was 161 percent of saturation and carbon dioxide (CO2 was at 26 percent of saturation. At Yampa River downstream from a diversion near Hayden, 16.3 miles downstream, the effects of photosynthesis were still dominant, though attenuated by reaeration and dilution with freshwater from the Elk River. About 37.2 miles farther downstream, at Yampa River below Craig, which is about 6.2 miles downstream from the Craig Waste Water Treatment Plant, the effects of photosynthesis increased slightly, and pH rose to 8.80. Respiration plus oxidation of organic matter became dominant at Yampa River at Deerlodge Park in Dinosaur National Monument, where pH was 8.51, dissolved oxygen concentration was at 109 percent of saturation, and CO2 was at 189 percent of saturation. Respiration plus oxidation of organic matter, though diminished, apparently extended to the mouth of the Yampa River.

Diurnal measurements on the Yampa River during August 2326, 1999, show that the effects of photosynthesis and respiration plus oxidation of organic matter decreased downstream with distance from the developed urban area in the eastern part of the basin. Larger night-time values of pH in Dinosaur National Monument at Deerlodge Park and at the mouth of the Yampa River indicate that source waters varied with respect to capacity for respiration plus oxidation and photo-synthesis, that photosynthesis was minor, and that pH was largely controlled by respiration plus oxidation of organic matter.

Synoptic sampling was repeated during March 13-16, 2000, when discharge was larger in response to late-winter melting of snow and ice at lower altitudes in the basin. Concentrations of nitrite plus nitrate were about 9 times greater in the Yampa River during March 2000 than during August 1999, and the largest increase (greater than 1,200 percent) was at Yampa River below Craig. At and downstream from Steamboat Springs, Colorado, pH at Yampa River sites averaged 8.85 during synoptic sampling in March 2000 compared to 8.70 in August 1999, with the 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 CO2 into and dissolved oxygen out of the river because of colder and deeper water and (2) slower rates of CO2 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.

Flow-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 195074 and 197599. These two periods were defined to represent the general periods of time before and after onsite measurements 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 197599. Dissolved concentrations of chloride, fluoride, and silica were significantly greater during 195074. Alkalinity and dissolved potassium concentration were not significantly greater during either period. The CO2 saturation factor was significantly greater during 195074 (median 10.2) than during 197599 (median 2.5). However, hypothetical equilibration of all samples with ambient atmospheric pressure of CO2 resulted in no significant difference in pH for the two periods. Therefore, the significantly greater measured pH values during 197599 cannot be attributed to the significant increase in concentrations of dissolved solids, calcium, magnesium, sodium, and sulfate, leaving decrease in the partial pressure of CO2 as the most likely cause.

Greater dominance of rates of respiration plus oxidation of organic matter (relative to rates of photosynthesis) during 195074 and (or) greater dominance of rates of photosynthesis (relative to rates of respiration plus oxidation of organic matter) during 197599 possibly contributed to the significantly smaller measured pH values at Yampa River near Maybell during 195074, 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, probably are less reliable than onsite measurements made after that date.

Table of Contents



Purpose and Scope

Description of Study Area

Sample Collection and Measurements


Interpretation of Data Collected for this Study

Summer 1999

Synoptic Sampling

Diurnal Measurements

Reconnaissance Measurements in Dinosaur National Monument

Winter 2000 Synoptic Sampling

Estimate of Maximum Potential Late-Afternoon pH in the Lower Yampa River Basin

Interpretation of Historical Data from Yampa river near Maybell

Trends in Dissolved Solids and Major Ions

Trends in Thermodynamic Properties

Causes of Historical Increase in Measured pH at Yampa River near Maybell


References Cited

Supplemental Information

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