WSP04002

• File Number: 8270.100
• Description: Colorado River Basin Water Quality/Salinity -- Misc Water Quality
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1974
• Author: USDOI
• Title: Colorado River Water Quality Improvement Program - Status Report
• Water Supply Pro - Doc Type: Report/Study

salts residing in soil and substrata. The salt
concentrating is due to the evapotranspiration of the
water carrying a finite salt load or the export of
high-quality water out of the basin. Generally, the
application of irrigation water results in increased salt
concentrations because of both loading and
concentrating. The evaporated or transpired water is
free of salt, hence, the ground.water return flows carry
the total salt burden. Under some conditions, however,
salts may be precipitated and stored in the soil. These
processes operate through the geochemical cycle
depicted in Figure 1. As water is evaporated from the
ocean or other free water surfaces, alterations in the
composition and concentration of the dissolved
contituents begin to occur and continue as the
concentration progressively increases during movement
through the cycle.

CHAPTER II. THE SALlNTY ISSUE

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The waters of the Colorado River system serve millions
of people in many ways. It is a vital link in sustaining
cities and farms, mines and industry, recreational space
and wildlife, and areas of great esthetic value to the
Nation. The water is used for irrigating crops,
producing energy, providing recreation, sustaining
livestock and wildlife, supporting industry, and
supplying the common daily needs of people. It is the
source of water for cities with in the basin and larger
metropolitan areas outside the basin. A large variety of
crops, providing a needed diversity to the national diet,
are irrigated by its waters both within and outside the
basin. But the concentration of dissolved solids in the
river, now among the highest of the great rivers of
North America, is increasing. The increase will further
impair the usefulness of the water.

Rising high in the Rocky Mountains, the Colorado
River flows for most of its length through qrid and
semiarid regions of the United States and Mexico. The
great river and its tributaries have cut into mountains
and plateaus, formed alluvial valleys, carved
magnificent canyons, and produced a highly productive
delta. In the process, its waters accumulate the solution
products of erosion and weathering-from headwaters
to mouth, a distance of nearly 1,400 miles, the salinity
progressively increases.

The river drains 242,000 square miies-1/12 the
conterminous area of the United States-and 2,000
square miles in Mexico. The basin includes parts of
seven states-southwestern Wyoming, western
Colorado, parts of Utah, Nevada, New Mexico, nearly
all of Arizona, and a small section of California. From
this area, it carries a salinity burden of 10 million tons
annually. Each square mile above the Grand Canyon
contributes an average of about 70 tons of salt to the
river each year. The salt loading arises from both
natural and manmade sources. The water is used and
reused several times along its length causing the volume
of available water to decrease while the salinity
increases. The potential demands on the river exceed
its dependable supply. The major overriding issue on
the Colorado River involves the interrelated matter of
inadequacy of water supply to meet all proposed uses
and deteriorating water quality.

SOURCES OF SALINITY

In any river system, salinity concentrations arise from a
salt loading effect and a salt concentrating effect. The
salt loading may be regarded as the pickup of salt due
to mineral weathering and dissolution of the soluble

While this process adds a large variety of dissolved
matter to the water, only 10 elements made up about
99 percent of the dissolved constituents. These are
hydrogen, sodium, magnesium, potassium, calcium,
silicon, chlorine, oxygen, carbon, and sulfur. The
elements occur in solution as various ions, molecules,
or radicals. The major part of the dissolved
constituents in Colorado River water is made up of the
cations: calcium, magnesium, and sodium; and the
anions: sulface, chloride, and bicarbonate. These, plus
minor amounts of other dissolved constituents, are
commonly referred to as salinity, "salt," dissolved
solids, or dissolved mineral content.

Studies of the operation of the geochemical cycle
suggest that about half the salinity concentration in the
waters at Hoover Dam is derived from natural sources
and the remaining half from manmade uses. The order
of magnitude from greatest to least is: natural sources,
irrigation, reservoir evaporation, out-of-basin export,
and municipal and industrial sources.

SALINITY LEVELS

HISTORIC AND MODIFIED CONDITIONS

The long,term 1941-1970 historic average annual
salinity concentration of the Colorado River at its
headwaters is less than 50 mg/1. At Imperial Dam, the
last major diversion point in the Un ited States, the.
concentration is 757 mg/I. Modifying this historic
condition to reflect all upstream existing projects to be
in operation for the full period 1941-1970 would again
show a concentration of less than 50 mg/l at
headwaters and a value of 851 mg/l at Imperial Dam.

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