Water law and policy
Overview
This page summarizes the laws and policies governing the allocation and management of water in the Colorado River Basin. These laws and policies bound the decision space for the basin’s water managers and thus influence how scientific research and related technical practice might inform current and future management, such as with the upcoming renegotiation of the 2007 Interim Guidelines. The evolving scientific understanding of the basin (e.g., climate change impacts on streamflows) can also lead to reconsideration of assumptions underpinning existing policy, providing impetus for policy change.
Prior appropriation
Much of the water in the West is allocated and managed according to the doctrine of prior appropriation. The prior appropriation system was developed in California in response to the need for water for gold mining and is tailored to watersheds with highly variable seasonal flows and limited water resources. It differs dramatically from the Riparian Doctrine of water law that dominates in the eastern United States. The four guiding principles of the prior appropriation system are:
1. Water rights are established by diverting water for a beneficial use. These water rights are usufructuary: they grant the ability to use the water--a public resource--but not to own it.
2. Earlier (senior) water rights have priority over later (junior) water rights during times of shortage and the order of water rights is maintained by a priority list. When a water user places a call on the river, all upstream water rights junior to the water user who placed the call are restricted until the call is removed. This principle is commonly referred to as “first in time, first in right.”
3. Water can be transported away from the stream for distant use, and ownership of land in the watershed is not mandated for use of water from the watershed.
4. Established water rights can be sold without changing the priority date.
The Law of the River
The “Law of the River” refers to the interlocking body of compacts, treaties, laws, regulations, and court decisions that guides water allocation and use in the Colorado River Basin among the seven basin states, Mexico, and sovereign Indian nations.
While the doctrine of prior appropriation still generally applies to the administration of water rights and use within each basin state, the overall effect of the Law of the River is that prior appropriation does not fully apply between states, or between the U.S. and Mexico, or between the U.S. or the states and tribal nations. In particular, many exceptions have been made to the stipulation that water rights are established by actually diverting water; starting with the Colorado River Compact of 1922, water allocations and rights have been granted well in excess of existing uses.
Colorado River Compact (1922)
The Colorado River Compact is the keystone of the Law of the River and allocates water between the two sub-basins: the Upper Basin and the Lower Basin, which are divided at Lee Ferry, located 20 miles downstream from Lake Powell. The Upper Basin (aka Upper Division) states are Wyoming, Colorado, Utah, and New Mexico (a very small part of Arizona is located above Lee Ferry). The Lower Basin (aka Lower Division) states are Arizona, Nevada, and California.
The negotiators of the Colorado River Compact assumed that the annual mean flow of the Colorado River at Lee Ferry was 17 or 18 million acre-feet (maf) per year of water based on the 20-30 years of gage records to that point. However, the long-term mean natural flow at Lee Ferry (1906-2021) has been 14.7 maf/yr, and from 2000-2021, the mean flow was only 12.3 maf/yr.
The Compact allocates 7.5 maf per year to the Upper Basin states and 7.5 maf per year to the Lower Basin states. The Compact also grants the Lower Basin the right to increase its water use by one million acre-feet annually and states that the Upper Basin “will not cause the flow at Lee Ferry to be depleted below an aggregate of 75,000,000 acre-feet for any period of ten consecutive years.” The Upper Basin has so far consistently met the terms of this non-depletion clause, albeit while using far less than its full 7.5 maf/yr allocation: about 4.5 maf/yr since 2015. While the Colorado River Compact of 1922 did not include allocations to Mexico, it stipulated that any future water obligations to Mexico would be shared by the Upper and Lower Basins equally.
Further allocation of water
While the Colorado River Compact of 1922 was clear in its division of water between the Upper Basin and the Lower Basin, it did not specify the allocations to each state. The allocations that were subsequently made or quantified to the states and Mexico are summarized in the table below.
| Who? | Sub-region | Allocation | Legal Basis |
|---|---|---|---|
| Wyoming | Upper Basin | 11.25% of available Upper Basin supply (1,043,000 of 7.5 maf) | Upper Colorado River Basin Compact (1948) |
| Colorado | Upper Basin | 51.75% (3,860,000 if 7.5 maf) | Upper Colorado River Basin Compact (1948) |
| Utah | Upper Basin | 23% (1,710,000 of 7.5 maf) | Upper Colorado River Basin Compact (1948) |
| New Mexico | Upper Basin | 11.25% (838,000 of 7.5 maf) | Upper Colorado River Basin Compact (1948) |
| Arizona | Upper Basin | 50,000 af | Upper Colorado River Basin Compact (1948) |
| Arizona | Lower Basin | 2,800,000 af | Boulder Canyon Project Act (1928) & Arizona v. California decree (1964) |
| Nevada | Lower Basin | 300,000 af | Boulder Canyon Project Act (1928) & Arizona v. California decree (1964) |
| California | Lower Basin | 4,400,000 (+ not more than 50% of surplus) |
Boulder Canyon Project Act (1928) & Arizona v. California decree (1964) |
| Mexico | n/a | 1,500,000 (+ 0.2 maf in years of surplus) |
U.S.-Mexico Water Treaty (1944) |
Tribal water rights
While the 1922 Colorado River Compact did not explicitly grant any tribal water rights, its Article VII says that the Compact should not be interpreted as affecting the federal obligations to the tribes that were established by the Winters v. U.S. (1908) Supreme Court decision. Starting in 1963, under many separate court decisions and settlement acts, a total of over 3 maf/yr of diversion rights to the Colorado River water have been quantified and allocated to 18 of the 30 tribes with reservations in the Colorado River Basin. Over half of these diversion rights by volume are allocated to three tribal nations: the Navajo Nation (NM), the Ute Indian Tribe (UT), and the Colorado River Indian Tribes (AZ).
Another three tribes receive Colorado River water under contracts with the federal government, but without formal quantification of their rights. One dozen tribes have outstanding claims to water rights that have not been quantified at all, and five tribes have partially quantified claims. Tribal water rights are quantified based on the “practically irrigated acreage” of tribal lands, per Winters (1908) and Arizona v. California (1963); however, tribes are able to use the water for purposes other than agriculture after it has been allocated.
Not all of these diversion rights are exercised each year; many tribes lack sufficient water infrastructure to fully utilize their rights. The water that is actually consumed under these diversions rights--which may be as little as half of what was diverted, depending on location and use--is counted as part of the overall allocation made to the state in which the reservation is located. Some of the water allocated to tribes is leased to non-tribal water users--agricultural, municipal, and industrial.