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The Colorado River Compact: How Water is Divided in the American West

An agreement from 1922 that divides the Colorado River's water among seven U.S. states and Mexico, shaping the arid West.

By Garret Merkley · Explainer · Jun 5, 2026
Branched from Prior Appropriation Doctrine: How Western Water Rights Actually Work
Quick take
  • The Colorado River Compact is a 1922 agreement allocating the river's water among seven U.S. states.
  • It divided the basin into Upper and Lower sections, each initially receiving 7.5 million acre-feet per year.
  • The Compact is the cornerstone of Western water law but is strained by drought and climate change.
  • It governs water supply for over 40 million people and vast agricultural lands in the American Southwest.

The Colorado River Compact is a foundational agreement signed in 1922 by seven U.S. states: Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. Its primary purpose was to prevent upstream states from monopolizing the river's water resources before downstream states could develop, effectively dividing the river’s available flow among them and establishing a framework for its management.

Dividing the Basin: Upper and Lower Allocations

The Compact divided the Colorado River Basin into two main sections: the Upper Basin (Colorado, New Mexico, Utah, and Wyoming) and the Lower Basin (Arizona, California, and Nevada). Each basin was initially allocated 7.5 million acre-feet (MAF) of water per year. This allocation was based on hydrological data from a period of unusually high river flows, a critical factor that contributes to today's challenges. A separate treaty in 1944 later allocated an additional 1.5 MAF per year to Mexico.

The Law of the River: A Complex Framework

The Compact serves as the cornerstone of what is known as 'The Law of the River,' a complex body of laws, treaties, court decisions, and operating guidelines. It mandates that the Upper Basin must deliver a minimum amount of water—specifically, 75 MAF over any 10 consecutive years—to the Lower Basin at a point known as Lee Ferry, Arizona. Within each basin, states then further subdivide their allocations, often relying on the Prior Appropriation Doctrine to assign individual water rights.

The Compact did not, however, specify how the water would be divided among the individual states within each basin, leaving those details to subsequent agreements and laws. For instance, the Upper Colorado River Basin Compact of 1948 further allocated water among its four member states, and the 1968 Colorado River Basin Project Act clarified Arizona's share in the Lower Basin.

The Colorado River Compact is critically important because it underpins the water supply for more than 40 million people and supports 5.5 million acres of agriculture across the American Southwest. Today, it matters more than ever as the original allocations, made during a wetter period, clash with modern realities of prolonged drought and climate change. The river’s flow has significantly declined, leading to severe shortages, dangerously low reservoir levels at Lake Mead and Lake Powell, and ongoing negotiations among states to reduce water use and avert a regional crisis. It highlights the tension between historical legal frameworks and current environmental pressures.

Key Numbers from the Compact
  • Signed in: 1922
  • U.S. States involved: 7 (CO, NM, UT, WY in Upper Basin; AZ, CA, NV in Lower Basin)
  • Initial allocation per U.S. basin: 7.5 million acre-feet (MAF) per year
  • Mexico's allocation (1944 treaty): 1.5 MAF per year
  • Current Challenge: River flows are often significantly less than allocated amounts.
Why is the Compact controversial today?
The Compact allocated more water than the Colorado River reliably provides, especially with climate change and persistent drought. This leads to chronic shortages and difficult disputes over which states must cut back their water use.
What is an 'acre-foot'?
An acre-foot is a standard unit for measuring large volumes of water. It's the amount of water needed to cover one acre of land to a depth of one foot, equivalent to roughly 325,851 gallons.
How does the Compact relate to individual water rights?
The Compact allocates water *between* states. Within each state's allocated share, individual water rights (often based on the Prior Appropriation Doctrine) then determine who gets to use that water for specific purposes like farming or municipal supply.
What happens if states cannot agree on water cuts?
If states cannot reach consensus on necessary water reductions, the federal government, primarily through the U.S. Bureau of Reclamation, has the authority to impose cuts, particularly on Lower Basin states, to protect critical reservoir levels and ensure minimum delivery obligations are met.
Explore further
Understanding the Prior Appropriation Doctrine in Western Water LawThe Role of Lake Mead and Lake Powell in Colorado River ManagementClimate Change and Its Impact on Western Water ResourcesWater Conservation Strategies for Arid Regions in the American WestThe 1944 Water Treaty: Mexico's Rights to the Colorado RiverNavigating Drought Contingency Plans for the Colorado River Basin

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