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

Evaluation of Trends in pH in the Yampa River,
Northwestern Colorado, 1950-2000
By Daniel T. Chaffin
Abstract
In 1999, the U.S. Geological Survey began
a study of pH trends in the Yampa River from near
its headwaters to its mouth. The study was
prompted by an apparent historical increase in
measured pH at the Yampa River near Maybell,
from an average of about 7.6 in the 1950's and
1960's to about 8.3 in the 1980's and 1990's. If
real, further increase could cause more frequent
exceedances of the Colorado water-quality stan-
dard of 9.0 and adversely affect aquatic life in the
Yampa River Basin, including Dinosaur National
Monument. The principal conclusion of this study
is that this apparent historical increase in
measured pH was caused mostly by changes in
measurement protocol.
Synoptic sampling during August 16-19,
1999, a period of relatively warm weather and
base flow, showed that late afternoon pH of the
Yampa River ranged from 8.46 to 9.20. The
largest pH (9.20) exceeded the Colorado water-
quality standard and was measured at Yampa
River above Elk River, about 1.8 miles down-
stream from the Steamboat Springs Regional
Waste Water Treatment Plant outfall, where
nutrient enrichment caused photosynthesis by
algae to dominate. Here, the dissolved oxygen
concentration was 161 percent of saturation and
carbon dioxide (CO2) was at 26 percent of satura-
tion. At Yampa River downstream from a diver-
sion near Hayden, 16.3 miles downstream, the
effects of photosynthesis were still dominant,
though attenuated by reaeration and dilution with
freshwater from the Elk River. About 37.2 miles
farther downstream, at Yampa River below Craig,
which is about 6.2 miles downstream from the
Craig Waste Water Treatment Plant, the effects of
photosynthesis increased slightly, and pH rose to
8.80. Respiration plus oxidation of organic matter
became dominant at Yampa River at Deerlodge
Park in Dinosaur National Monument, where pH
was 8.51, dissolved oxygen concentration was at
109 percent of saturation, and CO2 was at 189
percent of saturation. Respiration plus oxidation
of organic matter, though diminished, apparently
extended to the mouth of the Yampa River.
Diurnal measurements on the Yampa River
during August 23-26, 1999, show that the effects
of photosynthesis and respiration plus oxidation
of organic matter decreased downstream with
distance from the developed urban area in the
eastern part of the basin. Larger night-time values
of pH in Dinosaur National Monument at Deer-
lodge Park and at the mouth of the Yampa River
indicate that source waters varied with respect to
capacity for respiration plus oxidation and photo-
synthesis, that photosynthesis was minor, and that
pH was largely controlled by respiration plus
oxidation of organic matter.
Synoptic sampling was repeated during
March 13-16, 2000, when discharge was larger in
response to late-winter melting of snow and ice at
lower altitudes in the basin. Concentrations of
nitrite plus nitrate were about 9 times greater in
the Yampa River during March 2000 than during
August 1999, and the largest increase (greater
than 1,200 percent) was at Yampa River below
Craig. At and downstream from Steamboat
Springs, Colorado, pH at Yampa River sites aver-
aged 8.85 during synoptic sampling in March
2000 compared to 8.70 in August 1999, with the
Abstract 1