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250 W ~ ~ zoo ~ z J vOoC J ~ W v ~ a 150 °z~ J U ~ 100 OOC'3 UU~ 50 Z Q10 . Q7 `~~ 11 12 `. 5 6 - 1 4 7 ^1011 ^-,--- 3 5 6 ^ 8 ~-f------------- '~~-- ~'--------------f'-------- 9 12 2 r~4 f---------- 0' 230 220 200 180 160 140 120 100 80 60 40 20 DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES EXPLANATION --~--- Calcium at Yampa River site ^ Calcium at tributary site --~-- Alkalinity at Yampa River site Q Alkalinity at tributary site Note: number by symbol is site number in tables 1 and 3 to OaC O (J UQ ~ U ~~ za _~ } W F- J Z ~ J W Q ~ Y J Q Figure 4. Concentrations of calcium and alkalinity at Yampa River and main tributary sites, August 16-19, 1999. dissolved oxygen concentration that was 161 percent of saturation (relative to the concentration that would occur at equilibrium with the atmosphere)(fig. 5). Calculations using the thermodynamic-speciation program PHREEQC (Parkhurst, 1995) indicates that dissolved carbon dioxide gas (COZ) (fig. 5) was under- saturated at 26 percent of the concentration that would occur at equilibrium with the atmosphere. Dreyer (1982) describes the formation of algae (which represents microscopic and macroscopic plant composition) by photosynthesis: 106 C02 + 16 N03 + HP042- + 122 HZO + 18 H+ + (trace elements, energy) _ C106H2630~~oN16P1(algae) + 138 O2. (1) This equation explains the large pH value measured at site 3 by the uptake of dissolved C02, thus decreasing the concentration of carbonic acid (HZC03) dissolved in the water, and by direct uptake of H+ ions, which raises pH. Photosynthesis also explains the large oversaturation with dissolved oxygen. Equation 1 also shows that the formation of plant biomass depends on the availability of dissolved nitrogen and phosphorus, either of which can limit the ability of the reaction to proceed. The sample from site 3 had the largest measured concentration of dissolved phosphorus (0.06 mg/L) and the second largest (after site 1) measured concentrations of dissolved ammonia plus organic nitrogen (0.30 mg/L) and dissolved organic carbon (5.0 mg/L) of the main-stem and tribu- tary river sites. These constituents are derived from sewage effluent, livestock waste, excretions from aquatic organisms, and other natural sources. The effects of photosynthesis on pH and dissolved oxygen concentration were attenuated at Yampa River below diversion near Hayden (site 5) by reaeration in the approximately 16.3-mile reach down- stream from the SSRWWTP and by dilution with fresher water from the Elk River (site 4). However, the relatively large pH value at site 5 (8.70) was elevated by photosynthesis, as is indicated by oversaturation with dissolved oxygen (124 percent) and undersatura- tion with COZ (69 percent) (fig. 5). The Yampa River below Craig (site 6) is located about 6.2 miles downstream from the outfall from the Craig Waste Water Treatment Plant (CWWTP). This facility discharges about 1.9 ft3/s of treated sewage effluent during daylight hours (Robert Frazier, Craig Waste Water Treatment Plant, oral commun., 2000). At site 6, the effects of photosynthesis were only slightly greater than at site 5. Measured pH increased slightly to 8.80, the second largest measured value (after site 3), whereas dissolved oxygen was slightly more oversaturated (131 percent) and C02 was more undersaturated (59 percent) than at site 5 (fig. 5). The relatively minor effects of photosynthesis on water quality at site 6 compared to the effects at site 3 were (at least partially) due to the larger dilution of sewage INTERPRETATION OF DATA COLLECTED FOR THIS STUDY