7905

• UCREFRP Catalog Number: 7905
• Author: U.S. Department of the Interior.
• Title: Quality of Water, Colorado River Basin.
• USFW Year: 1995.
• USFW - Doc Type: Progress Report No. 17,
• Copyright Material: NO

Chapter 3

CAUSES AND IMPACTS OF SALINITY

CAUSES OF SALINITY

Nearly half of the salinity in the Colorado River System is from natural sources.
Saline springs, erosion of saline geologic formations, and runoff all contribute to this
background salinity. Irrigation, reservoir
evaporation, and municipal and industrial
(M&I) sources makeup the balance of the
salinity problem in the Colorado River Basin
(Basin). Figure 2 shows the relative amounts
each source contributes to the salinity
problem. The Environmental Protection
Agency (EP A)[l] estimated the natural
salinity at Hoover Dam was 334 milligrams
per liter (mg/L). In 1993, the salinity at
Hoover Dam was reported to be 660 mg/L, a
326-mg/L increase.

Natural (47%)

Reservoir (12%)

Figure 2.-Sources of salinity.

The natural or background salinity of the Colorado River has been increased by the
development of water resources in two major ways: (1) the addition of salts from
water use and (2) the consumption (depletion) of water.

The combined effects of water use and consumption have had a significant impact on
salinity in the Colorado River Basin. Concern over the damaging levels of salinity
prompted the Basin States and the Federal Government to adopt salinity standards
and an implementation plan to limit further increases in salinity that are discussed
later in chapters 5 and 6.

Agricultural Sources of Salinity

Irrigated agriculture is the largest user of water in the Colorado River Basin and a
major contributor to the salinity of the system. lorns[2] found that irrigated lands in
the Upper Colorado River Basin contribute about 3.4 million tons of salt per year
(37 percent of the salinity of the river). Irrigation increases salinity (figure 3) by
consuming water and by dissolving salts found in the underlying saline soils and
geologic formations, usually marine shales.

Deep percolation mobilizes the salts found naturally in the soils, especially if the
lands are over irrigated. Through salinity control practices, these contributions to the
river system can be dramatically limited, helping maximize the future beneficial uses
of the river.

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,

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