WSP05341

• File Number: 8278.100
• Description: Title I - Yuma Desalting Plant
• Basin: Colorado Mainstem
• Date: 12/23/1992
• Author: USDOI/BOR
• Title: Yuma Desalting Plant - Alternatives for the Interim Period
• Water Supply Pro - Doc Type: Publication

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the PRPU, combined with 15,000 af of water from drainage in the Yuma
Valley, would furnish 140,000 af of Mexico's 1.5 million acre-foot
annual entitlement for delivery at the Southerly International Boundary.

Additionally, as authorized by Title I, approximately 23,500 acres of
private, State, and State-leased lands have been acquired within the
5-mile zone. The purpose of these acquisitions is to limit development
and, thus, limit United States ground-water pumping to 160,000 af per
year, as required by Minute No. 242. Acquisitions were completed
in 1984.

SANTA CLARA SLOUGH

The Slough is a remnant of a much larger wetland complex which
disappeared as a result of the total diversion of the Lower Colorado
River. Prior to receiving flows from the Bypass Drain, the Slough was a
relat.ively small marsh of less than 100 acres. Constructing the Bypass
Drain and conveying WMIDD irrigation drainage into the Slough has
expanded the Slough's aquatic and wetland environment. The freshening
effect of the drainage water since 1976 has allowed the formation of
relatively secure habitat for many species of wildlife, both native and
introduced. Due to the unexpected length of construction, the bypassed
WMIDD irrigation drainage has expanded the open water and marsh habitat
of the Slough from a relatively small marsh to a currently estimated
10,000 acres.

Two of-the United States', federally endangered species, the
Yuma Clapper Rail bird and the Desert Pupfish, occur in the marsh.
By some estimates, one-fourth to one-half of all known Yuma Clapper
Rails inhabit the marsh. Desert Pupfish comprised from 80 to 90 percent
of the fish in the Slough area before Bypass Drain flows began. Since
then, many varieties of exotic fish have been transported by the Bypass
Drain. Pupfish numbers increased in the early years of Bypass Drain
flows. However, over time, the non-native fish may have somewhat
buffered the advantages of this additional habitat to pupfish. The
current status of pupfish is not known, although they are present.

Concern has been expressed that when the Desalting Plant begins
operation at full-scale capacity, the resulting combination of reduced
flows plus increased salinity will adversely impact the Slough and its
associated species. Under full-scale operation of the Desalting Plant,
it is expected that the water in the Bypass Drain would be reduced by
two-thirds from 183 ft3/s to 74 ft3/s, and the salinity could increase
from about 3,000 ppm to about 6,500 ppm.

However, the Yuma Desalting Plant is currently operating at one-third
capacity through 1995. During this period, the flows and salinity
levels reaching the Slough are not expected to significantly impact the
ecosystem. If total WMIDD drainage were reduced to 132,000 af per year,
operation of the Desalting Plant at one-third capacity would reduce the
flows through the bypass canal by about 20 percent and would increase
the salinity of the water reaching the Slough from the bypass canal by

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