WSP05944

• File Number: 8276.300
• Description: Lower Gunnison Basin Unit - Colorado River Basin Salinity Control Project
• State: CO
• Basin: Colorado Mainstem
• Water Division: 4
• Date: 9/1/1981
• Title: Final Report of the Lower Gunnison Salinity Control Study
• Water Supply Pro - Doc Type: Publication

I. SUMMARY

This USDA salinity control study was carried out under Public Law 93-320,
the Colorado River Basin Salinity Control Act (88 Stat. 266), dated June 24,
1974. Section 201(c) of the Act directs the Secretary of the Interior, the
Administrator of the Environmental Protection Agency, and the Secretary of
Agriculture to cooperate and coordinate their activities effectively to
carry out objectives of Title II of the Act as the basin states continue
developing their compact apportioned water. The Lower Gunnison Unit is one
of four irrigation source control units designated in the Act for
expeditious completion of a planning report as a means to implement the
salinity control policy adopted for the Colorado River.

The USDA has two primary purposes in the Lower Gunnison study: (1) to
determine the contribution of salt loading from the irrigated farm land and
privately owned upland areas; and (2) to determine the opportunity for
reducing salt loading through improvements in irrigtition systems and
practices, and reducing erosion and sediment delivery from the privately
owned uplands.

The scope of the study was limited to inventorying and analyzing current
onfarm irrigation systems and practices on about 107,000 acres of irrigated
land and 320 miles of off-farm canals and laterals. Results of these
analyses were expanded to be representative of the approximately 171,000
acres of irrigated land and about 485 miles of off-farm canals and laterals
in the Lower Gunnison and Uncompahgre River Valleys. Canals and laterals
that are within the Uncompahgre project area are being evaluated separately
by the Bureau of Reclamation and are not included in the 485 miles mentioned
above.

Coordination Of study activities leading to analysis of the salt loading
problem in the Lower Gunnison Unit was accomplished through a Multi-
Objective Planning Team under leadership of the Bureau of Reclamation.
Participants in this team effort included personnel from the Bureau of
Reclamation, Soil Conservation Service, U. S. Fish and Wildlife Service,
U.S. Geological Survey, Colorado Water Conservation Board, Colorado
Department of Health and the Delta County Soil Conservation District.

Mancos shale, a marine formation with high salt content, underlies most
of the valley (See Figure 111-2) and is the principal source of salt in the
Lower Gunnison Unit. Lenses of crystalline salt often are exposed during
excavation into shale. Because of the arid climate salts have not been
leached naturally and applying irrigation water to the soil greatly accele-
rates the leaching process.

The Gunnison River contributes about 1.4 million tons of salt annually
to the Colorado River. About 600,000 tons come from natural sources and
about 840,000 tons are attributed to irrigated agriculture which was first
introduced into the valley about 100 years ago. Most of the salt is leached
from the soil and underlying Mancos shale and carried to the river by deep
percolation from irrigation and by seepage from earthen ditches.

Improved management of irrigation water including land leveling, measur-
ing water onto the fields and lining onfarm and off-farm ditches shows the

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