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

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SEASONAL TRENDS ARKANSAS RIVER 1972 - 1174

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Figure 24.-Seasonal trends of dissolved copper, manganese, iron, and zinc in Arkansas
River, 1972-74.

the month of December 1975, there was an increase in
concentration toward the center of the reservoir from
both ends. During the same sampling period, similar
trends were observed for total hardness, zinc, lead, and
nickel concentrations, while the dissolved concentra-
tions of sulfate, potassium, and mercury decreased to-
ward the center. Manganese and iron increased toward
the inlet during November 1974, while just about
everything else decreased.

These observed trends would seem to arise from a
number of mechanisms involving several variables, in-
cluding:

1. The concentrative effect of evaporation from the
reservoir pool.

2. The varying quality of the water entering the
reservoir due to seasonal or source effects coupled
with a buffering effect of the reservoir.

3. The prevailing weather conditions (those at the
time of sampling and effects of recent runoffs
from local rain or snow).

The first effect mentioned would always have a tend-
ency to concentrate the dissolved solids, which would

predict decreasing concentrations moving toward the
inlet of the reservoir, which is the most commonly ob-
served trend.

The second effect is one in which water of higher or
lower concentration in dissolved solids is feeding the
reservoir. Water of lower concentration of ions feeds
the reservoir in the spring and early summer, creating a
dilution effect which would result in a decreasing con-
centration trend moving toward the inlet. However,
water feeding the pool in winter will normally contain
higher concentrations of dissolved ions, which will tend
to create an increasing concentration gradient moving
toward the inlet. Diverted water will have a similar
effect, depending on its quality.

The third effect is that of prevailing weather conditions.
Runoff from local rain will tend to feed the reservoir
with water containing relatively low concentrations of
various dissolved ions; whereas, runoff from snow, be-
cause it is slower, will have a higher dissolved ion con-
centration.

An inverse effect wou Id be expected for the suspended
loads being carried into the reservoir, i.e., rain, high;
snow, low. The phenomena such as observed in calcium
hardness curves (fig. 18) and other ion concentrations

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