FLOOD10381

• County: Pueblo
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Effects of Impoundment on Water and Sediment in the Arkansas River at Pueblo Reservoir
• Date: 5/1/1977
• Floodplain - Doc Type: Educational/Technical/Reference Information

tables 1 and 2 for the dissolved and suspended fractions
of the samples. The faf right-hand column of each
table in appendices A and B is the average and standard
deviation for each parameter for the sampling year that
appears in the table.

..

The tables in appendices A and B were produced from
computer printouts of the data. Because a consistent
format was needed to computerize the data, the values
listed do not in all cases reflect the proper number of
significant figures. Tables 1 and 2 also summarize where
the uncertainty appears and also give the approximate
magnitude of that uncertainty for each parameter.

Table4 (sediment data) was not generated by computer
and in all cases the proper uncertainty is reflected in
the number of significant figures displayed. All data in
table 4 are in units of mg/g (milligrams per gram) dry
weight. The values for parameters marked with aster-
isks were obtained by X-ray fluorescence spectroscopy.

RESULTS AND DISCUSSION

Seasonal Trends (Dissolved Fraction)

Figures 4 through 16 plot the values obtained for dis-
solved parameters at the inlet and outlet of the reser-
voir as well as the average value obtained for all sam-
ples collected in the reservoir pool as a function of
time. The most frequent seasonal trend observed is that
of maxima occurring during the winter months when
runoff is low, and minima occurring in the summer
months when the runoff is high, thereby producing a
maximum dilution effect.

During the summer it was noted that the inlet values
for most parameters were lower than for the outlet
or pool. The inlet values rose rapidly during the fall,
becoming higher than the pool average and outlet dur-
ing the winter and early spring months. This behavior
indicates that the reservoir is acting as a buffer zone,
somewhat leveling the normal seasonal trends observed
for the river.

AI

Obvious exceptions to the general trends described
above are the seasonal trends observed for temperature
and turbidity. The minimum for temperatures obvi-
ously occurs in midwinter and the maximum in mid-
summer. Turbidity maxima occur in late spring during
the highest runoff periods and are at a minimum during
the midwinter months. The anomalous trends of iron
(fig. 161. zinc (fig. 141. and manganese (fig. 15) are
discussed in those sections. Many of the trace metals
were at levels below or very near detectable limits of
measurement, making the observation of trends diffi-
cult and in many cases impossible.

"

Figure 23 plots the seasonal trends observed for hard-
ness, alkalinity, and sulfate in the Arkansas River (1972-
1974) before formation of the reservoir. The same gen-
eral trends were observed for the pre-impoundment
river as for the reservoir (1974-76) with an important
subtle difference. In the pre-impoundment river, max-
ima were obtained for those parameters during Decem-
ber, followed by a gradual decrease in concentration to
a minimum in June or July. In the reservoir a drop in
concentration was not observed until April or May of
the first year and was still high during the last sampling
during March 1976. These data would also seem to
indicate, as in the case of the evidence cited before,
that the reservoir is acting as a buffer to the seasonal
trends. However, it should be noted that during the
two impoundment sampling years, the inlet values for
these and other parameters also remained high until
April or May, somewhat negating the buffering argu-
ment based solely on the pre-, post-impoundment com-
parison. Other arguments, however, tend to reinforce
the buffering mechanism.

Surface and Spatial Trends (Dissolved Fraction)

Figures 17 and 18 are spatial plots of the average of
the temperature and calcium hardness at the various
sites moving from the outlet west to the inlet for the
months of March, July, and December for both sam-
pling years. Figures 19 through 21 plot the values ob-
tained at the surface versus site moving east to west for
selected parameters for the month of November 1974.
During July, the average reservoir temperature generally
declined moving toward the inlet, while during March
of each year the average temperature rose (fig. 18). In
the month of December the temperature fell moving
toward the outlet in 1974 and rose during 1975. Simi-
lar trends were observed for calcium hardness. During
July of both years, there was a slight decrease in cal-
cium hardness concentration moving toward the inlet
as there was for March 1975, I n March 1976, the reser-
voir pool was about constant with regard to calcium
hardness, while slightly higher at the inlet and outlet.
During December 1974, calcium hardness was about
constant over all sites, while in December 1975, a cur-
ious crown shaped curve was obtained with the lowest
values at the inlet and outlet and rising to a maximum
at the center of the reservoir. Trends observed for
samples collected at the surface (figs. 19 and 20) show
a slight decrease in the values moving east to west for all
the parameters listed except manganese, which showed
a sharp increase going east to west. An examination of
the data indicates that during most of the months of
sampling, slight increases or decreases in values do occur
for most parameters moving east to west over the res-
ervoir. In some cases there is a definite increase or de-
crease at and near the inlet (site F) of the reservoir. In
some cases (as in the case of calcium hardness) during

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