Law of the River – Building blocks for the past, present and future of the Colorado River

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collage of images from CAP canal history

Western water lore includes frequent references to the Law of the River. This refers not just to one law, but a compendium of sorts that includes compacts, treaties, federal laws, court decisions, a decree, contracts, agreements and regulatory guidelines. A review of the Law of the River traces how we got to where we are now related to how the Colorado River system is managed; serves as the basis for current discussions; and points us to where we’re going in the Colorado River Basin.

The Law of the River has evolved continuously over the past century, building upon itself, often providing solutions to rising issues. Each new building block is intertwined with all the former blocks, making the body of the Law of the River quite complex. You can see a full “master timeline” here.

Although there are dozens of touchpoints along the way, here are a few milestones to help make sense of what is happening now.

Colorado River Compact of 1922 – Nov. 24, 1922

  • First agreement to legally connect the states that share the Colorado River water supply
  • Designated the Upper and Lower Colorado River Basins, allocating 7.5 MAF/year to each
  • Defined Upper Division States and Lower Division States
  • Required the Upper Division States to deliver to the Lower Division States not less than 75,000,000 acre-feet over each consecutive rolling 10-year period, plus one-half of the Mexico Treaty delivery obligation.
  • Arizona refused to ratify the Compact until 1944

Boulder Canyon Project Act of 1928 – Dec. 21, 1928

  • Authorized construction of Hoover Dam
  • Apportioned Colorado River water among the Lower Basin states: 2.8 MAF/year to Arizona; 4.4 MAF to California; 300,000 acre-feet to Nevada

1944 U.S.-Mexico Water Treaty – Feb. 3, 1944

  • Apportioned 1.5 MAF/year of Colorado River to Mexico during normal years and specified that Mexico would share proportionately in times of surplus and in reductions due to “extraordinary drought”
  • Established the binational International Boundary and Water Commission to implement the Treaty

U.S. Supreme Court Decree: Arizona v. California – March 9, 1964

  • After a decade of deliberation, the U.S. Supreme Court delivered its opinion in favor of Arizona, upholding its designated 2.8 MAF/year share of mainstream Colorado River water
  • Effectively established the U.S. Secretary of the Interior as the Lower Basin “water master”

Colorado River Basin Project Act – Sept. 30, 1968

  • Authorized the Bureau of Reclamation to fund and construct the Central Arizona Project – learn more here
  • Created a junior priority in the Lower Basin for CAP water and for any new Arizona contracts entered after 1968 in times when insufficient mainstem Colorado River water is available to deliver a total of 7.5 million acre-feet to Arizona, California and Nevada
  • Directed the U.S. Secretary of the Interior to develop a plan for augmenting the Colorado River supply
  • Declared the Mexican Treaty delivery obligation a “national obligation”

2007 Interim Guidelines – Feb. 28, 2007

Drought Continency Plan Authorization Act – April 16, 2019

Lower Basin highlights (learn more about Arizona’s DCP Implementation Plan here):

  • Supplements the 2007 Interim Guidelines
  • Provides additional flexibility for the creation, storage and delivery of ICS
  • Created a “Tier Zero” at Lake Mead, requiring contributions to Lake Mead earlier, starting at elevation 1090’, and increased contributions at lower elevations
  • California agrees to make contributions to Lake Mead starting at elevation 1045’
  • Provided for the protection of Lake Mead elevation 1020’ – 500+ Plan

Upper Basin highlights:

  • Designed to minimize the risk of Lake Powell falling below elevation 3525’ and ensure compliance with the Colorado River Compact
  • Establish foundation for the storage of water in the Upper Basin as part of a demand management program that may be developed in the future

One of the upcoming milestones in the Law of the River will be the new guidelines that will take place when the 2007 Interim Guidelines expire in 2026. The Colorado River Basin states will play a leading role in developing these new guidelines.

The Arizona Department of Water Resources and Central Arizona Project have convened the Arizona Reconsultation Committee to provide input on Arizona’s participation and path forward in this process.