Water-Resources Investigation 95-4101
Title
Initial effects of Stagecoach Reservoir on discharge, water-quality
characteristics, and suspended-sediment loads in the Yampa River, northwestern
Colorado
Author
R. L. Tobin
Availability
Available from USGS, Earth Science Information Center, Open-File Reports
Section, Box 25286, MS 517, Denver Federal Center, Denver, CO 80225, USGS
Water-Resources Investigations Report 95-4101, 111 p, 32 fig.
Abstract
The construction and filling of Stagecoach Reservoir on the Yampa River during
1988-91 were done to enhance water management and to use local water resources.
To assess the initial effects of the reservoir on the hydrology of the upper
Yampa River, physical, chemical, and biological data were collected at a site
upstream (YR-1) during water years 1989-92 and a site downstream (YR-2) from the
reservoir during water years 1985-92 and at two sites in the reservoir during
1990-92. Annual suspended-sediment loads were determined for the Yampa River for
water years 1985-92, and sediment retention in Stagecoach Reservoir was
estimated. The initial filling of the 33,275-acre-foot reservoir proceeded
slower than expected because inflow from the Yampa River was about 50 to 73
percent of average during water years 1989-91. Secchi-disk measurements in
Stagecoach Reservoir ranged from 2.5 to 18 feet. Algal growth and sediment
transport during stormy weather decreased water clarity, and possible algal
grazing by zooplankton and sediment deposition improved water clarity. Water
temperature in the reservoir ranged from 0 to 22 degrees Celsius, and thermal
stratification was maintained during summer. Values of pH ranged from 7.2 in the
hypolimnion to 8.9 in the epilimnion. Changes in pH were related to
photosynthesis and respiration. Concentrations of dissolved oxygen in the
reservoir ranged from 0 milligram per liter in the hypolimnion to 13 milligrams
per liter in the epilimnion. Average 5-day biochemical-oxygen-demand rates
ranged from 0.33 to 0.46 milligram per liter per day. Oxygen production from
photosynthesis was greatest in the epilimnion; oxygen depletion from respiration
was characteristic in the hypolimnion. Near or above average inflow might
decrease the incidence of anaerobic conditions. Specific conductance in the
reservoir ranged from 414 to 520 microsiemens per centimeter at 25 degrees
Celsius, depending on the specific conductance of inflow from the Yampa River.
The water was a very hard, calcium bicarbonate type. Nitrogen input to the
reservoir was mostly as organic nitrogen that ranged in concentration from less
than 0.18 to about 1.0 milligram per liter. Concentrations of dissolved
phosphorus in the inflow of the Yampa River ranged from less than 0.01 to 0.06
milligram per liter. Decomposition of organic material and release of nutrients
from sediments under reducing conditions were probable causes for
dissolved-ammonia concentrations near the reservoir bottom to increase to
maximum values of 0.9 to 1.6 milligrams per liter as nitrogen during thermal
stratification in summer. Dissolved phosphorus also increased in the same
conditions to a range of 0.32 to 0.35 milligram per liter. Except for
concentrations of total recoverable manganese that ranged from 210 to 440
micrograms per liter near the reservoir bottom, most concentrations of 20 trace
constituents were measured at or near analytical detection limits. A total of
119 phytoplankton from 7 phyla was identified in Stagecoach Reservoir during
1990-92. Cyanophyta (blue-green algae) accounted for most of the cell counts.
Cyanophyta blooms of Aphanizomenon and Aphanocapsa developed during 1990-92, and
photosynthesis caused concentrations of dissolved oxygen to exceed 150-percent
saturation in the epilimnion. Diversity index values for phytoplankton ranged
from 0.05 to 3.06. Values of diversity index during the summer of 1992 indicated
that the community diversity of algae could be greatest in spring and least in
fall. All colony counts of fecal coliform bacteria in the reservoir during
1990-92 were less than criteria limits set by the State of Colorado. During
water years 1985-88 (preconstruction period), at a site on the Yampa River
downstream from the proposed damsite, and water years 1989-92 (post-construction
period), at a site upstream from the dam, annual loads of suspended sediment
ranged from 2,480 to 22,650 tons. The average annual suspended-sediment load for
these two sites was about 9,650 tons. The annual sediment displacement of
reservoir capacity was about 7 acre-feet, or about 0.02 of the original
reservoir capacity. During initial filling of Stagecoach Reservoir, ranges of
water temperature and specific conductance downstream from the reservoir were
decreased, and values of turbidity, pH, dissolved oxygen, fecal coliform
bacteria, and suspended-sediment concentrations decreased compared with inflow
data. Ranges of dissolved ammonia and phosphorus were 5 to 20 times greater than
the range from the base-line site on the Yampa River prior to dam construction
and the upstream control site after dam construction. Most downstream values of
dissolved oxygen and nutrients returned to near pre-impoundment ranges after the
reservoir had filled.
Keywords
Discharge, Sediment loads, Water-quality characteristics, Reservoirs, Limnology,
Water resources, Phytoplankton, Northwestern Colorado
State
CO
Watershed
14050001 14050002
Year
1995
Source
SWRA 1997
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