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<br />
<br />Diurnal measurements at Yampa River near
<br />Maybell (site 8 in fig, I) during August 23-24, 1999
<br />(Jig, 9), showed smaller effects of photosynthesis
<br />and respiration plus oxidation of organic matter than
<br />at sites 3 and 6, Values for pH peaked at 8,66 at
<br />1900 hours on August 23 and attained a minimum of
<br />8,52 at 0700 hours the next day (a small diurnal
<br />change of 0,14 unit), Dissolved oxygen peaked at
<br />132 percent saturation at 1500 hours on August 23 and
<br />attained a minimum of 86 percent saturation at
<br />0100 hours and 0600 hours the next day, Specific
<br />conductance ranged from 474 to 502 ~/cm,
<br />Diurnal measurements at Yampa River at Deer-
<br />lodge Park (site II in fig, I) during August 25-26,
<br />] 999 (fig, 10), showed little or no effect of photosyn-
<br />thesis on pH and dissolved oxygen saturation, The
<br />smallest pH values (8.45-8,51) were measured during
<br />daylight on August 25, and the largest pH values
<br />(8,59-8.60) were measured between the hours of 2200
<br />on August 25 and 0500 hours on August 26 (an
<br />inverted diurnal change of 0,15 unit), Dissolved
<br />oxygen concentration peaked at ] 06 percent of satura-
<br />tion at 1700 hours on August 25 and reached a
<br />minimum at 90 percent of saturation at 0700 hours the
<br />next day, with concentrations varying in the narrow
<br />range from 6,7 to 7.3 mg/L and remaining within
<br />5 percent of saturation for most of the measuring
<br />period, Specific conductance ranged from 555 to
<br />5711lS/cm, Cloudy weather to about 1330 hours and
<br />from about 1630 hours until after sunset on August 25
<br />probably suppressed photosynthesis, the presence of
<br />which was weakly evidenced by the pattern of dis-
<br />solved oxygen saturation. However, cloudy weather
<br />during daylight cannot account for the larger pH
<br />during night, which indicates a change in the chem-
<br />istry of the water flowing past site 1 L
<br />Diurnal measurements at Yampa River at mouth
<br />(Echo Park){site 12 in fig, 1) during August 25-26,
<br />1999 (fig, II), showed substantial effects ofphotosyn-
<br />thesis on dissolved oxygen concentration but not pH,
<br />Values of pH varied slightly from 8,60 to 8,70, with
<br />the largest values generally occurring between 2200
<br />and 0200 hours, Dissolved oxygen concentrations
<br />ranged from 6,8 to 8.4 mg/L, peaking at 118 percent of
<br />saturation at 1500 hours on August 25, which indi-
<br />cated that photosynthesis was occurring, and attained a
<br />minimum of 93 percent of saturation at 0700 hours the
<br />next day, which indicates slight respiration plus oxida-
<br />tion of organic matter. Specific conductance ranged
<br />from 581 to 591 ~/cm, Except for a brief sunny
<br />
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<br />period during midafternoon, cloudy weather on
<br />August 25 probably suppressed photosynthesis, As at
<br />site 11, larger pH during night indicates a change in
<br />the chemistry of the water flowing past site 12,
<br />
<br />Diurnal measurements on the Yampa River
<br />during August 23-26, 1999, showed a general down-
<br />stream decrease in the effects of photosynthesis and
<br />respiration plus oxidation of organic matter. From
<br />Yampa River above Elk River (site 3) to Yampa River
<br />below Craig (site 6) to Yampa River near Maybell
<br />(site 8), there was a decrease in maximum daily pH
<br />values (9,07 to 8,78 to 8,66), in diurnal fluctuation in
<br />pH values (1.15 to 0.61 to 0,14 units), and in
<br />maximum daily dissolved oxygen saturation (178 to
<br />140 to 132 percent); these trends show that eutrophica-
<br />tion of the river and the effects of photosynthesis
<br />decreased substantially upstream from site 8, In addi-
<br />tion, the downstream increase in minimum dissolved
<br />oxygen saturation (66 to 78 to 86 percent) indicates
<br />that respiration plus oxidation of organic matter
<br />decreased substantially between sites 3 and 8, The
<br />small diurnal fluctuation in pH values (0,14 unit) at
<br />site 8 indicates that respiration plus oxidation of
<br />organic matter during afternoon hours produced CO2
<br />at about the same rate as it was consumed by photo-
<br />synthesis; this interpretation is supported by the 89
<br />percent of saturation with CO2 at site 8 during
<br />synoptic sampling on August 16, 1999 (fig, 5). During
<br />night, when photosynthesis does not occur, respiration
<br />plus oxidation of organic matter, although slower
<br />because of cooler temperature, would have continued
<br />to produce CO2 and lower pH.
<br />
<br />Cloudy weather during most of the day when
<br />diurnal measurements were made at sites 11 and 12
<br />made interpretation of those measurements more prob-
<br />lematic. However, larger pH values at those sites
<br />during night (figs, 10 and 11) indicates that (1) source-
<br />water chemistry varied with respect to capacity for
<br />respiration plus oxidation of organic matter and photo-
<br />synthesis and (2) photosynthesis was minor and pH
<br />was largely controlled by relative rates of CO2 produc-
<br />tion caused by respiration plus oxidation of reactive
<br />organic matter, which would have been greater during
<br />warmer daylight hours, lowering pH. The second point
<br />is supported by oversaturation of CO2 in river water at
<br />sites 11 and 12 during synoptic sampling (fig, 5).
<br />However, this observation does not discount the possi-
<br />bility that variation in source-water chemistry also
<br />affected diurnal pH.
<br />
<br />14 Evaluation of Trends in pH In the Vamps River, Northwestern Colorado,1950-2000
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