The turbulent Colorado River is one of the most heavily regulated rivers in the world.

Serving as the “lifeline of the Southwest,” the Colorado River provides water to 40 million people and more than 4 million acres of farmland in a region encompassing some 246,000 square miles.

The Colorado falls some 10,000 feet on its way from the Rocky Mountains to the Gulf of California. From its headwaters northwest of Denver, the 1,450-mile long river and its tributaries pass through parts of seven states: Arizona, California, Colorado, New Mexico, Nevada, Utah and Wyoming and is also used by the Republic of Mexico.

The Colorado River runs for about 300 miles through Marble Canyon and the Grand Canyon. The powerful forces of erosion have carved through 2 billion years of the earth’s geologic history at the Grand Canyon. Layers of limestone, sandstone, shale, granite, and schist make up the Grand Canyon’s rock sequences. 

The canyon’s depth varies from almost a vertical mile along the South Rim to almost 6,000 feet at other locations. The canyon’s width ranges from an average of 10 miles across near Grand Canyon Village but may be as wide as 18 miles across from rim to rim.

Along the way, almost every drop of the Colorado River is allocated for use.

The Colorado River Basin is also home to a range of habitats and ecosystems from mountain to desert to ocean.

Colorado River Overview

The Colorado River has been tapped for use by humans for almost 1,500 years.

Today, more water is exported from the Colorado River Basin than any other river basin in the United States.

Water is diverted over the Continental Divide to supply Denver and other Front Range cities. Water is diverted in Utah to the Salt Lake Valley, in New Mexico to the Rio Grande Basin to serve Albuquerque, in Wyoming to serve Cheyenne and in California to the southern coastal plain of Los Angeles and San Diego.

Seven western states, the federal government, Mexico and 23 American Indian tribes share water rights to the water that flows 1,450 miles from the headwaters of the river in Colorado’s Rocky Mountain National Park to the Gulf of California. (For a more in-depth look at the various Colorado River stakeholders, see the Layperson’s Guide to the Colorado River).

The states comprise two basins: the Upper Basin consists of Colorado, New Mexico, Utah and Wyoming. The Lower Basin consists of Arizona, California, and Nevada. Lee Ferry is the dividing point between the Upper and Lower Basin.

As part of its diverse environments, the Colorado River boasts more than 30 fish species found nowhere else in the world. However, 50 percent of all native fish and in the Colorado Basin have either gone extinct are or considered vulnerable.

The river itself was originally muddy, brown and seasonally warm (the source of its name Colorado) but is now clear and cold due to dams and reservoirs.

The Colorado River was the last major area of the 48 contiguous states to be explored. In 1869, an expedition led by John Wesley Powell first explored and mapped the Colorado and Green rivers.

Recreation is also a significant part of the Colorado River system. The Colorado River is used by millions of people annually for activities ranging from rafting to snow sports and generates billions of dollars in revenue.

Colorado River Agreements

Colorado River water is shared by states, the federal government, American Indian tribes and Mexico, resulting in many compromises, interstate compacts, a U.S. Supreme Court decree and an international treaty.

In 1905, the Colorado River broke through a series of dikes, flooding an ancient seabed in the Imperial Valley and forming the Salton Sea. Subsequently, in 1922 the Colorado River Compact divided the water among the seven Western states with an allocation formula, apportioning 7.5 million acre-feet to each basin. As part of the agreement, courts established that California could use no more than 4.4 million acre-feet of water per year, plus half of any officially declared surplus.

In 1944, a U.S.-Mexico treaty resulted in an annual 1.5 million acre-feet allocation to Mexico.

The 2003 Quantification Settlement Agreement incorporated an Imperial Valley to San Diego County water transfer. This was followed by a 2007 agreement among the seven Colorado Basin states that established  a new water release formula between Powell and Mead lakes to help meet increased demand for water.

In 2012 the U.S. and Mexico signed Minute 319. Minute 319 establishes new rules for sharing Colorado River water through a five-year pact. Mexico, with limited storage capacity, may now store some of its Colorado River water in Lake Mead. In exchange, should a shortage be declared in the Lower Basin, less water will be sent to Mexico.

A continuation of Minute 319 called Minute 323 was finalized in September 2017. The agreement provides a continuous flow of water to the Colorado River Delta and expands the restored habitat area to from 1,700 acres to 4,300 acres. Mexico will continue to store water in Lake Mead and both governments will provide funding and other resources for research projects along the border and throughout the region.

Minute 323 requires that the U.S. contribute $31.5 million to conservation projects in Mexico focused on improving infrastructure. These projects are expected to save about 200,000 acre-feet of water each year. The money will come not only from the U.S. government but from the Metropolitan Water District of Southern California, Southern Nevada Water Authority, Imperial Irrigation District and Central Arizona Water Conservation District. In return for their funding, these water agencies will receive a portion of the saved water. In addition to funding for conservation projects, the U.S. government and nongovernmental agencies will fund $18 million for the habitat restoration and monitoring.

The idea of a Lower Basin Drought Contingency Plan (DCP) emerged in 2013 as an overlay to the 2007 shortage-sharing agreement. After the intense drought of the early 2000s, stakeholders had breathed a sigh of relief with a high flow on the river in 2011, only to be followed by the lowest consecutive years on record. Suddenly, the possibility of Lake Mead dropping to dead pool (when the water level is so low that it cannot drain by gravity through Hoover Dam’s outlets) didn’t seem far-fetched.

The DCP calls for the Lower Basin states to commit to voluntary water reductions to preserve Lake Mead and avoid the mandatory cutbacks associated with a shortage declaration by the federal government. Once agreed to by the states, the DCP requires congressional approval.

Colorado River and Current Issues

Since the 1990s, many programs have been underway to boost endangered species and restore some riverine habitat. Both wildlife and the environment have been impacted by water diversion and development such as dams.

Since 2000, the Colorado River Basin also has experienced drought, which has included some of the driest conditions on record. Hoover Dam and Glen Canyon Dam have both helped the Colorado Basin weather the drought, though their combined water storage has declined by 50 percent. The drought has led to tremendous disparity between high and low flows in the last decade.

Looking ahead, some experts predict climate change will cause the Colorado River Basin to become drier and warmer with less snowfall and runoff reducing water supplies. That prospect, combined with future projections of continued urban growth, has generated significant concern over how the Colorado River will meet future water demands.

With these issues in mind, in 2012 the Bureau of Reclamation published the Colorado River Basin Water Supply and Demand Study to help provide a road map for the future.

The study forecast a significant gap between available supply and the expected demands of a growing population by 2060. Specifically, the study said the mean natural flow of the river at Lee Ferry would decrease by about 9 percent, along with a projected increase in both drought frequency and duration as compared to the observed historical and paleo-based scenarios. Meanwhile, the population in the seven Colorado River Basin states is expected to be as many as 77 million people by 2060.

In 2018, the Colorado River Basin Ten Tribes Partnership Tribal Water Study was released. That study described how tribes use their water, how future water development could occur and the potential effects of future tribal water development on the Colorado River system. The study identified challenges related to the use of tribal water and explored opportunities that provide a wide range of benefits to both Partnership Tribes and other water users.

Another 2018 study, the U.S. Global Change Research Program’s Fourth National Climate Assessment, concluded that Earth’s climate is changing rapidly compared to the pace of natural variations that have occurred throughout its history, with greenhouse gas emissions largely the cause.

Much of what the assessment expects to happen in the Colorado River Basin depends on several variables, including changes in water demand and the rate of greenhouse gas emissions. According to the assessment, under certain scenarios, higher temperatures would cause more frequent and severe droughts in the Southwest, including megadroughts — dry periods lasting 20 years or longer.

Projected substantial reductions in snowpack and increased rain, earlier runoff and warmer late-season stream temperatures combined with an increasing population in Southern California, which imports most of its water, would increase the probability of future water shortages, according to the assessment.