Eric Kuhn, with some help from Anne Castle and myself, has taken a useful dive into what was known in the 1940s as Congress was considering the treaty between the United States and Mexico regarding how to share the waters of the Colorado River.

Drawing on the analysis of Colorado’s Royce Tipton, Eric draws out a theme we explored in our book Science Be Dammed – the gap between the scientific understanding of the day and the hydrologic reality we must now face as we try to manage a shrinking river.

Tipton, representing the state of Colorado, made a compelling case that there was enough water, including return flows from Lower Colorado River water use, to meet a 1.5 million acre foot allocation to Mexico without impairing US users.

California, already growing accustomed to living off the surplus of other water users’ unused apportionments, opposed the deal. Any water allocated to Mexico would cut into that surplus.

(California’s) major Colorado River projects were either complete, like the All-American Canal, or nearing completion, such as Metropolitan Water District’s Colorado River Aqueduct. In the 1930s, California agencies had signed contracts with the United States for 5.36 maf of Hoover Dam water, but under the 1928 Boulder Canyon Project Act, they only had a “senior” right to 4.4 maf. Thus, over 900,000 af of its water supply was reliant upon surplus water being available. California’s primary concern was that, together with further development by the other basin states, the treaty would significantly reduce or eliminate the surplus water available for use within the United States. California did not want to be limited to 4.4 maf per year.

Tipton’s hydrology was based on a wetter climate than we have today. It also was based on the idea of “salvage by use” – that water use in, for example, Arizona, should be apportioned based on the impact of that use on the main stem of the river, not on how much was being consumed at the point of use. This would be then, and remains today, a critical point of unresolved water law.

Tipton’s hydrology was based on studies completed by the Bureau of Reclamation as a part of its comprehensive development study of the basin. The period of record was 1897-1943, with a reconstructed natural flow (the flow that would have been in the stream absent human modifications) at the international boundary with Mexico of 17.75 maf per year (16.3 maf per year at Lee Ferry). Additionally, Tipton told the Committee that because of salvage by use (the reduction of natural losses as the flow of the river is reduced through consumption) a minimum of another 1.25 maf/year would be available for use in the United States. The major portion of his estimated salvage by use water would be generated on Arizona’s Gila River. Tipton believed that Mexico’s consumptive use in 1943 was approximately 1.8 maf/year, more than what the treaty would guarantee. He emphasized that an estimated 8 to 10 maf/year was then currently flowing into Mexico and because the Hoover Dam power operation stabilized the river, Mexico could substantially increase its irrigation use. He was reinforcing the core argument by the supporting states: the treaty would limit Mexico to less water than it was currently using by about 300,000 af per year, and without a treaty, in the future Mexico might acquire more water through international arbitration.

Key to Tipton’s winning argument in support of the treaty was the belief that the agreement would constrain what might otherwise be increasing Mexican demands on the river.

Eric also found a wonderful bit of business from Tipton that echoes in today’s Colorado River Basin arguments over what constitutes “hydrologic shortage”:

[A]s a member of the engineering profession I will admit that engineers have made mistakes many times. One type of mistake that has been made in the West has been recommendations to irrigate areas larger than available water supplies would permit. Some of these errors were made on account of lack of stream flow records. Most overexpansion, however, has resulted from the lack of control of development under our western water laws. In most states no one can be denied the right to appropriate the waters of a stream; and the only control is the administration of the use of the water so appropriated by the system of priorities. So that we have many, many systems that have only flood rights on our streams, which is due to no one’s fault.

We conclude with some advice about what might be done next:

While the Colorado River Basin is currently focused on the negotiations between the Upper and Lower Division States over the future management of the river, the post-2026 operating rules, sooner than later the United States and Mexico will have to negotiate the successor to Minute 323. Both Minute 323 and the 2007 Interim Guidelines expire at the end of 2026. Given the current stalemate between the divisions over the future operations of the river, both hydrologic and political uncertainties loom large.

Rather than waiting for the Basin States to resolve their differences, which may ultimately require lengthy interstate litigation, Mexico and the United States may want to consider a parallel and alternative approach. They should consider abandoning complicated approaches that set annual deliveries based on how much water is being used by the Lower Division States or Lake Mead storage levels and instead negotiate an agreement based on giving Mexico a fixed percentage of the river’s natural flow (probably at Lee Ferry). By doing so, they could set an example for a long-term solution that works for the entire basin.

Royce Tipton and the Hydrology of the 1944 Treaty with Mexico.