WSP10396

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 1/1/1964
• Author: USGS
• Title: General Effects of Drought on Water Resources on the Southwest
• Water Supply Pro - Doc Type: Report/Study

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DROUGHT IN THE SOUTHWEST, 1942-56

at a relatively constant rate, In the reach between Mal-
agR and Red Bluff the weighted average concentration
of dissolved solids increased by more than 1 ton per acre-
foot in each of the water years 1944-M and. by more
than 3 tons per acre-foot in the years 1952-54 inclusive.
Figure 12 provides a graphic. comparison of the
weighted average chloride concentration, in equivalents
per million, at each station for the 20 years: small dif-
ferences in the wet years 1941-43, moderate differences
in most other years, and large differences in 1952-54 and
195'7 when average streamflow was least, In other
words, this comparison suggests that the chloride con-
centration is inversely related to streamflow.
The relation of mean annual chloride concentration
to discharge of Pecos River near Red Bluff is shown in
figure 13. On the basis of this chloride rating curve,
a reasonably accurate estimate can be made of the
weighted average chloride content dnring any year in
which the mean annual discharge is known, Such a
rating curve could be developed for any gaging station
where a substantial part of the load of dissolved solids
is contributed at a relatively constant rate, as for ex-
ample from the ground-water sources near Malaga

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Bend, Less than half the sodium chloride in the
river near Red Bluff has come from the Malaga
Bend area; the rest has come from the 19,200-square-
mile d,'ainage area upstream from Malaga, The small
dispersion of points from the curve in figure 13 sug-
gests that the dissolved solids throughout this drainage
area may be contributed to the river at relatively con-
stant rates. The variations in streamflow cause varia-
tions in dilution of this saline contribution,
Variations in streamflow similarly cause variations
in dilution where streams enter salt-water bodies,
Coastal streams characteristically enter the ocean over
a "wedge" of salt water. In times of minimum flow
this wedge may invade the stream channel for several
miles, and in floods it is driven out to sea, At any point
along the lower reach of the channel, therefore, varia-
tions in streamflow may cause variations in chemical
quality, Within the Southwest drought area, as de-
limited in this report, few streams flow perennially into
the ocean, and for these few we do not have records of
the effect of the ocean upon the quality of the river
water, However, such effects are measured in Trinity
River at Liberty, Tex., where tides create backwater

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50 100 500 1000
MEAN ANNUAL DISCHARGE, IN CUBIC FEET PER SECOND

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FIGURE 13.-DJscharge-chlorJde relationsblp, Pecos River near Red Blu1f, N. Mex. (water years 1938-55).