WSP12647 ‎(2)‎

• File Number: 8282.400
• Description: Colorado River Operations and Accounting - Deliveries to Mexico
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 6/1/1999
• Author: Environmental Defense Fund
• Title: A Delta Once More - Restoring Riparian and Wetland Habitat in the Colorado River Delta - Environmental Defense Fund - 06-01-99
• Water Supply Pro - Doc Type: Report/Study

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ENVJRONJVfENlj\t DEFENSE FUND

Figure 4. Water Flows at the Southern International Boundary, 1997
(Valdes-Casillas et at., 7998a)

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Date (1997)

AGRICULTURAL WASTEWATER

Delta wetlands with significant conservation interest
also survive on agricultural wastewater. Seventeen
agricultural drains from the Mexicali Valley flow into
the Rio Hardy/Colorado River system, carrying an av-
erage annual volume of 51,000 acre-feet (6.33 x 107 m3)
(Valdes-Casillas et aI., 1998a). Another 125,000 acre-feet
(1.5 x 108 m3) of mildly saline (3000 parts per million)
agricultural wastewater pumped from Arizona's
Wellton-Mohawk Irrigation District is delivered to
Mexico at the Southern International Boundary. This
water is disposed in the eastern delta after travelling 48
miles (77 km) in a concrete canal called the Main Outlet
Drain Extension (MODE). MODE water joins about
25,000 acre-feet (3.1 x 107 m3) of agricultural wastewater
(from the Riito drain) to support la Cienega de Santa
Clara.

La Cienega de Santa Clara was once part of a Colorado
River channel that ran along the edge of the Sonoran
Mesa. As the channel shifted westward, this shallow
depression (formed by a branch of the San Andreas fault)
nearly dried up. In 1973, before MODE water arrived,

the wetland covered only 490 acres (200 ha), fed by
artesian springs and the Riito drain (Glenn et al., 1996).
After 1977, when the MODE water started emptying into
the Santa Clara pepression, flows supported wetlands
of up to 50,000 acres (20,000 ha). In 1993, floods on the
Gila River required the MODE to be closed for repairs
for about eight months. As a result, wetlands in la
Cienega de Santa Clara decreased in size to 2700 acres
(1100 ha). With the restoration of flows, the marsh
returned to its former dimensions within five months
(Glenn et aI., 1996; ZengeI et aI., 1995). [See Box 2.]

Agricultural wastewater can change ecosystem health
since it tends to affect the concentration of pollutants,
salts, and minerals. High levels of selenium, for example,
are found in many delta areas that receive wastewater,
and selenium is known to affect birds and other wild-
life. Although agricultural wastewater has been a fairly
constant source for la Cienega de Santa Clara, over the
years the MODE canal has carried less water and its
salinity has declined. This is due to lower pumping rates
and lower groundwater salinity. Wastewater flows from

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