The Department of State to the Mexican Embassy 80

Memorandum Regarding the Proposed International Water Treaty, United States and Mexico

colorado river

The memorandum of the Ambassador of Mexico, dated March 19, 1942, has had careful consideration. Pending the completion of still further studies being pursued with the purpose of ascertaining in the near future whether alternate formulas may be developed, the following observations are made.

The proposal of the United States for the delivery of Colorado River water to Mexico was predicated upon the confirmation and protection of uses in Mexico existing prior to the completion of Boulder Dam, based upon the unregulated flow of the river, and also upon a recognition of the rights of the seven Colorado River Basin States in the United States, as evidenced primarily by the Colorado River Compact among the States.81 The United States has recognized this Compact in the Act of Congress known as the Boulder Canyon Project Act.82 The Compact referred to made a primary allocation for consumptive use within the seven basin States.

This initial allocation of 16,000,000 acre-feet does not represent the maximum quantity which can be put to beneficial use in the United States. It was based, rather, upon what was then considered a firm annual supply, leaving to the future the disposition, among the States, of any additional quantities which might then be found to exist. An exhaustive survey, designed to determine the extent to which the water of the Colorado River may be beneficially used in the United States, and the available supply, is now in progress by agencies of the United States Government. This survey, however, may not be completed for several years.

Based upon the best data presently available, the total virgin flow of the river is estimated at 18,000,000 acre-feet per annum on the average, leaving an estimated average quantity of 2,000,000 acre-feet per year to take care of reservoir losses and for future allocations in the United States. This water can all be beneficially used in the United States. Projects in operation and under construction in the [Page 562] lower basin of the United States at the present time will use 9, 140,000 acre-feet of Colorado River water per year. This is 640,000 acre-feet more than the firm allocations of the Colorado River Compact to the lower Basin States. These projects do not comprehend the total possibilities of the lower basin.

In addition it must be borne in mind that we are here dealing with average figures which do not take into account extended periods of low run-off such as have been recently experienced, where American developments would necessarily be deprived of sufficient water if any substantial amount was guaranteed to Mexico. It is quite possible that in years of abnormal run-off 2,000,000 acre-feet of water or more could be delivered to Mexico without deprivation to lands in the United States, even after ultimate developments in the United States. In the average year, however, the amount that could be so delivered would be very much less than this figure, and during dry cycles much less than 1,000,000 acre-feet would be available without serious deprivation to American interests.

Mexico’s estimates of the water supply and that amount which will be available for ultimate use were undoubtedly based on early figures which have been shown by later surveys to be inaccurate. When the Colorado River Compact was negotiated in 1922 it was assumed that total water production in the basin was about 19,000,000 acre-feet. Sixteen million acre-feet were allocated by the Compact. Subsequent stream flow records have indicated that the 1922 estimate was too high. In this connection it is of interest to note that as gauging facilities have been improved, the estimates of virgin flow of the river have tended to decline, so much so as to place in some doubt the present estimate of 18,000,000 acre-feet. The total water production of the basin, including the Gila River, during the low run-off period, 1930–1940, averaged only about 14,500,000 acre-feet per year, or 1,500,000 acre-feet per year short of the Compact allocations. If such a 10-year cycle occurred at the end of the 50-year amortization period provided for Boulder Dam and appurtenant facilities, it is estimated that the release from Lake Mead would be only about 8,500,000 acre-feet per year and this would be made possible only by drawing the contents of the reservoir down 17,000,000 acre-feet during the 10 year period.

Furthermore, no account has apparently been taken in the data cited in the Ambassador’s memorandum of March 19, of reservoir evaporation losses, which are estimated as being in excess of 1,000,000 acre-feet annually, thus materially reducing the amount of water available for distribution, either to the United States or to Mexico.

Existing projects below the dam will require 7,800,000 acre-feet of this annual release of 8,500,000 acre-feet without taking into consideration [Page 563] additional units of projects already planned. In fact, studies of releases at Boulder Dam corresponding to the period 1930–1940, inclusive, show that in only three out of eleven years would Mexico have been supplied with the requested amount of 2,000,000 acre-feet after supplying the lower Basin projects under construction. Even this indicated surplus would diminish as future contemplated construction in the United States was realized. It is thus apparent that any waters allocated to Mexico over and above return waters present in the river at the international boundary must operate to restrict proportionately the ultimate development within the United States.

In the ten-year period prior to 1935, when Boulder Dam was placed in operation, the average area cultivated in Mexico was 152,000 acres. If the diversion duty is taken as 4.5 acre-feet per acre per annum the corresponding use of water would have been 684,000 acre-feet. The year of maximum use during this period was 1925, when 217,000 acres were cultivated. Expanded uses in Mexico since 1935 were due to, and made possible by, the construction of Boulder Dam and its appurtenant works, thus controlling and regulating the theretofore erratic flow of the stream. It should not be over-looked that actual water shortages occurred on several occasions between 1920 and 1935, hence the Mexican computations regarding surplus water must refer to the situation created by that Dam. This is a factor that cannot be ignored in determining the beneficial uses to which Mexico has established a right and precludes the use of the period subsequent to 1935 in making this determination.

The quantity that was proposed to be delivered to Mexico by the United States, therefore, is more than sufficient to provide for the maximum area which Mexico had cultivated prior to the construction of Boulder Dam, particularly considering the fact that the delivery was to be a controlled one, subject to agreed schedules. That ultimate uses in the United States will not always leave 2,000,000 acre-feet for Mexico was virtually admitted in the memorandum of July 22, 1941 of the Ambassador of Mexico;83 for therein it was stated that “the volume determined (2,000,000 acre-feet) will normally constitute a surplus from the American uses”. It will be recognized at once that no developmental program could safely be based upon a stated amount of water when that supply is only normal or average. Furthermore, as has been shown above, it would be only in years of abnormally high run-off that 2,000,000 acre-feet could be delivered to Mexico after full development in the United States.

  1. See memorandum of conversation with the Mexican Ambassador, supra. This memorandum is not filed with the memorandum of conversation but is presumably the one handed to the Ambassador.
  2. The Colorado River Compact assigned 7,500,000 acre-feet per year to the Upper Basin and the same amount to the Lower Basin, with the right, however, to increase this amount by 1,000,000 acre-feet per year.
  3. Approved December 21, 1928; 45 Stat. 1057.
  4. Not printed.