9431

• UCREFRP Catalog Number: 9431
• Author: Chafin, D. T.
• Title: Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000.
• USFW Year: 2002.
• USFW - Doc Type: Denver.
• Copyright Material: NO

Another parcel of water with initially greater but
decreasing salinity (specific conductance, fig. 12) was
measured during September 1-3. On September 1, pH
values increased from 8.43 to 8.68. The initial pH of
8.43 probably resulted from a larger concentration of
reactive organic matter associated with the more saline
water. This water possibly represented initial flushing
of salts and organic matter from the land surface by the
30-minute rain at 2000 hours on August 30. Downriver
measurement of decreasing specific conductance
during September 1-3 is consistent with the arrival of
fresher water that was progressively less affected by
such flushing.
The stabilization of pH in the narrow range
8.66-8.77 after 1025 hours on August 31 (for the first
parcel of water) and after 1205 hours on September 1
(for the second parcel) supports the hypothesis that pH
between sites 11 and 12 was mostly the result of
downstream depletion of reactive organic matter (the
concentration of which varies with source water) and
gradual venting of oversaturated, dissolved CO2.
Using results of synoptic samples collected on August
18 (site 11) and August 19 (site 12), water measured
during this reconnaissance trip, if equilibrated with
atmospheric PCp ,would be expected to have a pH of
about 8.75 (fig. 6~. Actual measurements indicate that
pH value was usually smaller and that respiration plus
oxidation of organic matter, not photosynthesis, was
dominant during daylight in this reach of the river-a
conclusion consistent with the preceding interpretation
of diurnal measurements at sites 11 (fig. 10) and 12
(fig. 11). Shading of much of the river by the canyon
walls probably suppressed photosynthesis.
No perennial creeks were observed during this
reconnaissance trip. Only one flowing creek was
measured at 1455 hours after rain on the morning of
September 1 about 19.2 miles from the mouth of the
river; this creek, which was flowing at about 0.2 ft3/s,
had a temperature of 25.1° C, a specific conductance
of 909 µS/cm, and a pH of 8.25. After the hard,
13-minute rain at 1152 hours on the morning of
September 1, waterfalls were observed to flow briefly
off the canyon walls.
Several perennial springs flow between sites 11
and 12. On August 31 at 1900 hours, a spring on the
south bank of the river 25.2 miles from the mouth was
measured to have a temperature of 15.7° C, a specific
conductance of 1,465 µS/cm, and a pH of 7.58.
Discharge of this spring, which built algae terraces,
was estimated to be from 0.004 to 0.01 ft3/s. Another
spring with similar discharge was observed on the
north bank of the river about 25.0 miles from the
mouth, but measurements were not made because of
approaching darkness. The only other perennial spring
observed during the reconnaissance trip was Warm
Springs (on the north bank of the river, 4.05 miles
upstream from the mouth), which was flowing at about
6 to 8 ft3/s at 1155 hours on September 3, 1999.
Measurements indicated a temperature of 16.2° C, a
specific conductance of 1,466 µS/cm, and a pH of
8.04.
Steele and others (1978) measured pH values
from 8.4 to 8.8 (with a mean of 8.6, as measured and
by concentration of H+ ions) and dissolved oxygen
concentrations from 7.4 to 8.0 mg/L (90 to 108 percent
of saturation) at five sites on the Yampa River between
Deerlodge Park and the river's mouth during August
17-19, 1976. They measured dissolved oxygen satura-
tions of 103, 106, and 108 percent between 1700 hours
and 1900 hours on the 3 days of measurement, indi-
cating minor photosynthesis. The range and means of
pH values measured by Steele and others (1978) are
essentially identical to those measured during the
reconnaissance trip for this study, which was
conducted when the season and discharge were
similar. Dissolved oxygen saturations in 1976 gener-
ally are similar to the diurnal and synoptic measure-
ments made for this study. Similarities in pH and
dissolved oxygen saturation measured by Steele and
others (1978) and measurements for this study indicate
that the Yampa River in Dinosaur National Monument
was not substantially different 23 years after the earlier
study with respect to the effects of photosynthesis on
water quality.
Winter 2000 Synoptic Sampling
Data-collection activities during winter 2000
consisted of synoptic sampling at nine Yampa River
sites and three tributary sites (fig. 1). Water-quality
data resulting from this sampling are presented in table
3 in the "Supplemental Information" section at the
back of this report.
During synoptic sampling during March 13-16,
2000, discharge at the nine Yampa River sites was
64 percent larger on average than discharge during
synoptic sampling during August 16-19, 1999. The
increase in discharge between the two sampling
periods generally increased downstream from
18 Evaluation of Trends in pH in the Yampa River, Northwestern Colorado, 1950-2000