WSP11975

• File Number: 8146.100
• Description: Pueblo Dam - Project Description
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1986
• Author: US DoI BoR
• Title: Physical Chemical and Biological Characteristics of Pueblo Reservoir Colorado 1985 - 1986
• Water Supply Pro - Doc Type: Report/Study

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tion in the epilimnion is partially disrupted downstream
from transect 3 as the reservoir surface cools and mixes
with the underlying water downstream, This process of
fall turnover continues into September as air tempera-
tures decrease substantially and can continue into
October until the deeper parts of the reservoir generally
are well mixed. Underflow of the Arkansas River helps
maintain stratified conditions upstream from transect 3
or 4 throughout the fall and winter.

Specific-Conductance Stratification and
Mixing Patterns

Variations in specific conductance in Pueblo
Reservoir are a function of the specific conductance
and initial routing of the Arkansas River and the ther-
mal stratification and mixing patterns within the reser-
voir. The specific conductance of the Arkansas River
upstream from Pueblo Reservoir varies inversely with
discharge. Specific conductance of the river water is
lowest in May through early July when discharge is rel-
atively large because of snowmelt runoff in the upper
basin, Specific conductance gradually increases with
decreasing discharge during the late summer and fall.
Matching of observed reservoir specific-conductance
values with similar values of antecedent specific con-
ductance of the Arkansas River inflow provides further
information on the effect of thermal stratification on the
initial routing of inflow into the reservoir and mixing
patterns within the resen'oir. The determination of
stratified or mixed conditions with respect to specific
conductance is qualitative; stratification is indicated by
a change in specific conductance with depth; well-
mixed conditions are indicated by a uniform or nearly
uniform specific-conductance profile,
The specific-conductance profiles for December
1986 and March through October 1987 (fig. 5) illus-
trate the typical temporal and spatial patterns in specific
conductance that were measured from 1985 through
1989. These patterns represent those commonly
observed in Pueblo Resen'oir throughout the year.
Specific-conductance stratification patterns are not as
well defined as thermal-stratification patterns. With
respect to specific conductance, the reservoir generally
is well mixed in the winter and early spring (December
and March) (fig. 5). The April and May profiles indi-
cate specific-conductance stratification is very weak
throughout the reservoir. Specific-conductance stratifi-
cation becomes well defined in June as underflow
becomes the dominant direction of inflow routing, and
inflow specific-conductance values decrease at the time
of peak snowmelt runoff. Early in the summer, the spe-
cific conductance of inflow from the river is relatively

low because of snowmelt runoff; therefore, the under-
flow results in smaller conduotance values in the
hypolimnion. As summer progresses, the inflow spe-
cific conductance in the Arkansas River increases as
discharge decreases; as a result, the underflow results in
larger specific-conductance values near the reservoir
bollom from July through October. Fall turnover mixes
the reservoir downstream from about transect 4 during
August through September. Generally uniform
specific-conductance values in the upper 40 to 70 ft of
the downstream part of the reservoir during the late
summer and fall indicate these areas are mixing during
this time because of fall turnover. Underflow of the
Arkansas River maintains relatively strong specific-
conductance stratification upstream from about
transect 4 during the autumn. Relatively stable
specific-conductance values in the river inflow and
mixing within the reservoir help maintain nearly uni-
form, well-mixed conditions from late autumn through
the spring throughout much of the resen'oir.

Reservoir Residence Times

Residence or retention time is defined as the time
necessary for the volume of water in a reservoir to be
replaced by inflowing water or the time necessary for
the volume of water in a reservoir to be drained by out-
flow, When the reservoir is well mixed, the residence
time can be calculated by using the following equation:

T=V/Q

(I)

where
T = residence time, in days;
V = reservoir volume, in acre-feet; and
Q = reservoir outflow, in acre-feet per day.

Theoretical residence times for Pueblo Resen'oir
can range from a few weeks to more than a year (fig, 6),
When the reservoir is thermally stratified, the residence
time, as calculated from the above equation, does not
accurately represent the actual residence time of water
entering the reservoir because of various mixing and
circulation patterns that occur within the reservoir,
Therefore, the equation for estimating theoretical resi-
dence time is valid from about October through March
or April, although there is some degree of thermal
stratification present upstream from about transect 3
during part of this time, Less than 7 percent of the total
volume of the reservoir is located upstream from
transect 3 when storage is at the top of the conservation
pool; therefore, the effects of thermal stratification and
underflow in this region on reservoir residence times is
small. As a result of the relatively strong thermal strat-

PHYSICAL CHARACTERISTICS 15