7409

• UCREFRP Catalog Number: 7409
• Author: U.S. Department of the Interior.
• Title: Quality of Water, Colorado River Basin.
• USFW Year: 1991.
• USFW - Doc Type: Progress Report No. 15,
• Copyright Material: NO

Chapter 4

Causes and Impacts of Salinity

CAUSES OF SALINITY

The natural or background salinity of the
Colorado River has been changed by the
development of water resources in two major
ways: the addition of salts and the depletion of
water. One of the original studies by Ioms [2]
on the addition of salts showed that irrigated
lands in the Upper Colorado River Basin
contribute about 3.4 million tons of salt per year.
This source of salt is one of the two major factors
increasing salinity in the Colorado River. The
other major factor is the consumptive use of
water. The consumptive use reduces the
dilution of-both natural sources of salt and the
new sources of salt caused by irrigation and
other developments.

Natural Sources of Salinity

Flow and quality records reveal that along
certain reaches of the Colorado River,large
increases in salt loads occur that cannot be
attributed to irrigation or other development
related activities. These increases are mainly
due to natural diffuse sources and saline
springs. Very little information was obtained
prior to irrigation, making it difficult to identify
\ the magnitude of specific natural sources of
,salinity in the Colorado River Basin.
\

Natural diffuse sources are those sources of salt
that occur gradually over long reaches of the
rivel' system. Salt pickup occurs over large
surface areas, from underlying soils, and from
stream channels and banks. Salt pickup is
difficult to identify, measure, or control. Diffuse
sources contribute the largest overall share of
the salts to the Colorado River. The natural salt
load for the Colorado River at Lees Ferry,
Arizona, was estimated to be about 5.3 million
tons per year [2,3]. Natural point sources are
mainly saline springs where the contribution of

salt and watel' is easily identified, issuing from
single or concentrated sources.

Agricultural Sources of Salinity

Salt balance conditions exist when the amount of
dissolved solids carried oft'the land is equal to
the amount added. Pickup of salt as used in this
report represents an unbalanced condition
shown by the increase of salt load in the return
flow over the totaIload in the applied water.
Salt pickup attributed to irrigation is that
additional amount which occurs as a result of
irrigation and does not include the amount
resulting from natural sources.

Irrigation development in the Upper Basin took
place gradually from the beginning of settlement
in about 1860 but was hastened by the purchase
of land from the Indians in 1873. About
800,000 acres were being irrigated by 1905.
Between 1905 and 1920, the development of
irrigated land increased at a rapid rate, and by
1920, nearly 1.4 million acres were being
irrigated. The development then leveled oft', and
increase since that time has been slow because
of physical and economic limitations on the
availability of water. About 1.5 million acres
were irrigated in 1980.

Irrigation development began in the Lower
Basin at about the same time as in the Upper
Basin but was slow due to the difficulty of
diverting from the Colorado River with its
widely fluctuating flows. Development of the
Gila area began in 1875 and the Palo Verde area
in 1879. Construction of the Boulder Canyon
Project in the 1930's, and other downstream
projects since that time, has provided for a
continued expansion of the irrigated area. In
1970, an additional 21,800 acres were irrigated
by private pumping either directly from the
Colorado River or from wells in the flood plain.
In 1980, nearly 400,000 acrEls were being