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1,000 U~ ~ Zw ~ o_ UZaw ~w~U ~ H ~ (n ~ W 500 Ui7~CZC'3 ~Uw~ U~U~ ~ Z N a° ~ 11 12 _. ^ $ y ----------- ~~ ___--t---------^ 3 6 _--- `~-~~ 5 _____-f---~- 2 ~'t---- ^4 200 180 160 140 120 100 80 60 40 20 DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES 0' ' 230 220 EXPLANATION - f -Specific conductance at Yampa River site ^ Specific conductance at tributary site Note: number by symbol is site number in tables 1 and 3 Figure 3. Specific conductance at Yampa River and tributary sites, August 16-19, 1999. Williams Fork at mouth (site 7), with a specific conductance of 527 µS/cm, and Little Snake River above Yampa River (site 10), with a specific conduc- tance of 809 µS/cm. Evaporation from the river channel, irrigation return flows, sewage effluent, and inflow of saltier ground water are other potential contributors to the downstream increase in salinity. Downstream trends in concentrations of calcium and alkalinity (fig. 4), important controls on pH and on the capacity of the waters to dissolve or precipitate calcite (CaC03), were similar to trends for specific conductance (fig. 3). Concentrations between Yampa River below Stagecoach Reservoir (site 1) and Yampa River at Steamboat Springs (site 2) were diluted by fresher water in tributary creeks. Downstream from Yampa River above Elk River (site 3), concentrations of calcium and alkalinity were substantially diluted by freshwater from the Elk River (site 4). From Yampa River below diversion near Hayden (site 5) to Yampa River at mouth (site 12), concentrations of calcium increased to 40 mg/L and alkalinity increased to 140 mg/L of alkalinity as CaC03, primarily because of inflow of more alkaline water from the Williams Fork (site 7) and the Little Snake River (site 10). At Yampa River sites, pH (fig. 5) ranged from 8.46 to 9.20; pH ranged from 8.08 to 8.70 at the three tributary sites, which were sampled during early after- noon before pH would be expected to peak because of photosynthesis. The smallest measured pH value in the Yampa River (8.46) was at site 1, below Stagecoach Reservoir, only about 0.25 mile downstream from the dam spillway. Because this water was released from substantial depth in the reservoir, pH had little poten- tial to be affected by the cumulative effects of photo- synthesis. However, water from this site had the largest measured concentrations of dissolved ammonia (0.02 mg/L as N), dissolved nitrite plus nitrate (0.10 mg/L as N), dissolved ammonia plus organic nitrogen (0.36 mg/L as N), dissolved organic carbon (5.4 mg/L as C), and the second largest measured concentration of dissolved phosphorus (0.05 mg/L as P) at Yampa River and tributary-river sites. In addition, the rocky streambed was covered with aquatic vegeta- tion. The largest pH value (9.20), measured at Yampa River above Elk River (site 3), exceeded the Colorado water-quality standard of 9.0 for the upper Colorado River Basin. Site 3 is about 1.8 miles downstream from the sewage outfall from the SSRWWTP. This facility discharges about 3.9 ft3/s of treated sewage effluent during summer daylight hours (David Jarvis, Steamboat Springs Regional Waste Water Treatment Plant, oral commun., 2000), indicating a fortyfold dilution of sewage effluent at site 3 during sampling. The river channel exhibited abundant attached algae and occasional clumps of entrained algae, indicating photosynthesis induced by nutrient enrichment from sewage effluent. The large pH was accompanied by a 8 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000