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9,3 9,2 9,1 en I- 9,0 Z ::J ci a: 8,9 <( Cl Z ~ 8,8 en ~ :C 8,7 c. 8,6 8,5 --.-- ,,0. -~- Measured pH Simulated pH at equilibrium with atmospheric carbon dioxide Simulated pH at equilibrium with atmospheric carbon dioxide and calcite Note: number by symbol is site number in tables 1 and 3 6 "'....-............ //// -----____ 8 9 11 -..----------11I0",,,,,,,,,,,12 8 1/ /" ..0."""'" ';P, .. I I I I / \ -",- \ _..-....-- 11./- 8 9 6_--------~------A~~------~ .3 II " " II I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I \.._-- : 3 11I/ ", I.Q 5 2C:P 'o" I . / 2 , ~ // .../' 3... _" ...............2 .JIfII&-, 1 -........, 1 Q,,\ \ 5",..,,0'" 0' 8.4 230 220 12 120 40 o 200 180 160 140 DISTANCE FROM MOUTH OF YAMPA RIVER, IN RIVER MILES 20 100 80 60 Figure 6, Measured and simulated pH at Yampa River sites, August 16-19,1999, Furth(lr hypothetical simulations using PHREEQC to allow samples collected from Yampa River sites tq precipitate enough calcite to attain equi- librium with that mineral further decreased pH values between 0,13 and 0,31 unit (compared to pH at equi- librium with atmospheric Peo ) (fig, 6), These simula- tions indicate that pH for Yam~a River water would be restricted to the narrow range from 8,42 to 8,50 if it was in equilibrium with calcite and atmospheric CO2, Diurnal Measurements Measurements at Yampa River above Elk River (site 3 in fig, 1) during August 23-24, 1999, indicated strong diurnal fluctuations in pH and dissolved oxygen concentration (fig, 7) as a result of dominance by photosynthesis during daylight and by respiration plus oxidation ofiorganic matter during night. Values for pH ranged from a peak of 9,07 at 1500 hours in the afternoon of August 23 to a minimum of 7,92 at 0300 hours the next morning (a diurnal change of LIS units), Dissolved oxygen concentrations peaked at 178 percent of saturation at the same time pH peaked and attained a minimum of 66 percent of satu- ration at the time of minimum pH, (Because the degree of saturation of dissolved oxygen is dependent on water temperature, minimum and maximum concen- tration of dissolved oxygen does not exactly corre- spond to minimum and maximum concentration,) Specific conductance remained relatively constant at 331 to 343 J,lS/cm over the 23-hour period, The abrupt decrease in pH value from 9,07 at 1500 hours to 8,84 at 1600 hours apparently was caused by a IS-minute cloudy period between those measurements; this temporary inhibition of photosynthesis probably prevented pH from reaching its maximum potential that afternoon at this site, a conclusion supported by a pH of 9.20 at 1600 hours on August 18, 1999, Diurnal measurements at Yampa River below Craig (site 6 in fig, 1) during August 23-24, 1999 (fig, 8) also indicated substantial effects of photosyn- thesis and respiration plus oxidation of organic matter, although not as strongly as at Yampa River above Elk River, Values for pH peaked at 8,78 at 1800 hours on August 23 and attained a minimum of 8,17 at 0600 hours the next day (a diurnal change of 0,61 unit), Dissolved oxygen concentration peaked at 140 percent of saturation at 1600 hours on August 23 and attained a minimum of 78 percent of saturation at 0600 hours the next day, Specific conductance ranged from 346 to 380 IlS/cm, 12 Evaluatio~ of Trends in pH in the Vamps River, Northwestern Colorado, 1950-2000