WSP02914

• File Number: 8278.400
• Description: Title I - Mexican Treaty
• Basin: Colorado Mainstem
• Date: 2/1/1963
• Author: USDOI/BOR
• Title: Special Studies - Delivery of Water to Mexico
• Water Supply Pro - Doc Type: Publication

Chapter II

Title I History and Provisions

I'

Title I History

In 1944, the United States and Mexico
signed a treaty requiring the United States
to deliver 1.5 million acre-feet of Colorado
River water to Mexico annually. The treaty
did not address the salinity of the delivered
water.

In 1961, the salinity of the delivery to
Mexico increased sharply from a range of
about 700 to 920 parts per million (ppm) to
about l,340ppm total dissolved solids
(TDS). In some months, the salinity
exceeded 2,500 ppm. 1

The increased salinity was caused by (1) the
discharge to the Colorado River of saline
irrigation drainage pumped from newly
constructed wells in the Wellton-Mohawk
Irrigation and Drainage District (WMIDD)
in Arizona; (2) a reduction in excess
Colorado River flows to Mexico, resulting
from the construction and closure of
Glen Canyon Dam; and (3) construction
of Painted Rock Dam, which significantly
reduced the less saline Gila River flood-
flows to the Colorado River below
Imperial Dam.

.

In November 1961, Mexico filed a formal
protest with the United States, ('lRiming
that the increased salinity was damaging
crops in the MeJlicali Valley, a violation of
international law. In response, the United
States began modifying WMIDD irrigation
drainage pumping and river operations in

J

1963 and 1964. In 1965, the United States
reached its first salinity agreement with
Mexico, Minute No. 218 ofthe International
Boundary and Water Commission (a 5-year
agreement that was later extended 2 years).
Minute No. 218 authorized the construction
and operation of the Main Outlet Drain
Extension (MODE), which allowed WMIDD
irrigation drainage to be discharged either
above or below Morelos Dam, Mexico's
principal diversion structure. (See
frontispiece map.)

After the MODE was constructed, about
40,000 acre-feet of irrigation drainage was
bypassed (discharged below Morelos Dam)
each year. The bypassed waters, which
were not considered part of Mexico's treaty
deliveries, were substituted with waters
from upstream storage. By the end of 1971,
these bypass operations and a gradual
improvement in the salinity ofWMIDD
irrigation drainage reduced the average
annual salinity of Mexico's deliveries to
about 1,245 ppm. Monthly averages varied
from 1,150 ppm to almost 1,500 ppm.

To maintain salinity below about
1,240 ppm, Mexico later asked the United
States to bypass an additional 40,000 to
75,000 acre-feet ofWMIDD irrigation
drainage each year. The additional amount
bypassed was considered part of Mexico's
treaty deliveries. The resulting average
annual salinity of the treaty deliveries was
reduced to about 1,160 ppm, but only by
reducing Mexico's usable deliveries.

1 Salinity refers to the salt content of eolutions containing dissolved mineral salto, generally measured as total
dissolved eolids in parts per million.

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