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The Colorado River flows past Lees Ferry, Arizona, on May 29, 2021. The river travels more than 1,400 miles from its headwaters in western Colorado to the Pacific Ocean. (AP Photo/Ross D. Franklin, File)

The Colorado River provides water for millions of people, including Coloradans from the Western Slope to the Eastern Plains. But much of the river system is overallocated, its waters are overused and its flows are shrinking. 

“It’s not a rosy picture. We’ve been in a drought for a very long time,” said Kevin Reidy, senior state water efficiency specialist with the Colorado Water Conservation Board, the state’s top water agency. “Really, what we’re looking at is aridification across the state, across the Southwest.”

What’s happening with the river is complicated, so The Colorado Sun is breaking down the basics.

Colorado River Explained. We’re answering your questions about the Colorado River. Send them our way and stay tuned for more!

How long is the Colorado River? 

The Colorado River starts as snowmelt north of Grand Lake in Rocky Mountain National Park and flows more than 1,400 miles — longer than the drive from San Diego to Seattle.

In Colorado, the river parallels Interstate 70 from Dotsero to Grand Junction before leaving the state. Then it passes through Utah, Lake Powell, the Grand Canyon and Lake Mead near Las Vegas. One section, through the Grand Canyon, is considered the most endangered waterway in the U.S. 

After Lake Mead, it heads down the California-Arizona border into northwestern Mexico. The river’s natural course ends in the Gulf of California, but since the early 1960s, when Glen Canyon Dam impounded the river near Page, Arizona, it has rarely reached the Pacific Ocean.

What is the Colorado River Basin?

Like other river drainage basins, also called watersheds, the Colorado River Basin is an area of land that collects the precipitation within it and drains it into a common outlet, the Gulf of California.

The enormous basin covers about 246,000 square miles and seven U.S. states: Arizona, California, Colorado, Nevada, New Mexico, Wyoming and Utah. It includes two Mexican states, Baja California and Sonora, and the lands of 30 federally recognized tribes.

The Colorado River’s tributaries include some of Colorado’s most well-known local rivers, like the Yampa, White, Eagle, Roaring Fork, Gunnison, Animas and Dolores. Some of the river’s major tributaries that pass through other states include the Green, San Juan, Little Colorado, Gila and Virgin.

On average, about 170 million acre-feet of precipitation falls over the Colorado River Basin annually, but only about 10% of it reaches streams and is available for use. One acre-foot supports about two families of four to five people for one year.

The basin is also mind-bogglingly old: It began forming about 40 million to 70 million years ago when two tectonic plates collided, forming the Rocky Mountains. That means it could be as old as the Tyrannosaurus rex. Most researchers agree the Colorado River established its present course about 6 million years ago, which is around the time that humans’ first known common ancestor may have appeared.

Why is the Colorado River system important?

Without the Colorado River, life in the American Southwest would not be what it is today — it’s that important. The river system supplies about 40 million people with water, helps provide food for the nation and supports impressive biodiversity. 

The Colorado River system has supported ecosystems and Indigenous communities since time immemorial. In the 1800s, settlers arrived in the basin’s arid landscapes where irrigation was the only way to grow a crop. They established the current way of using the river’s waters — water law, ditches, tunnels and dams.

Now, the river’s economic value is estimated to be $1.4 trillion per year. It irrigates about 5.5 million acres of agricultural lands and fuels a multibillion-dollar agricultural industry that produces 90% of the nation’s winter vegetables. Agriculture uses at least 75% of the basin’s water supply.

The basin’s rivers and streams are the reason major cities like Las Vegas, Phoenix and Los Angeles have been able to grow to their current size. In Colorado, the river system contributes 40% of the state’s water supply. More than 530,000 acre-feet of its flow is pumped east to Front Range cities, like Denver, Colorado Springs, Pueblo, Boulder, Fort Collins and Broomfield.

The basin provides water for hydroelectricity generation, industries and recreation. The banks of the Colorado River also support about 65% of the species in the West, which include many endangered species.

Is the Colorado River drying up?

The Colorado River might not dry up completely, but there’s a good chance it won’t always provide enough water for the millions of people who depend on it.

Much of the Colorado River Basin is in an arid to semiarid climate. Not only is the river prone to natural annual fluctuations in supply, but over time, it has experienced drought periods that can last decades. Previous research shows the current drought in the river basin is the driest period on record stretching back 1,200 years.

The Colorado River’s annual flow is also sensitive to environmental factors like rising temperatures, which are linked to increasing concentrations of greenhouse gases like carbon dioxide in the atmosphere.

Research shows the region has warmed by 2.7 degrees Fahrenheit since 1880, at the beginning of the fossil fuel industrial age. Between 2000 and 2021, the river basin lost 10 trillion gallons of water — roughly equal to the capacity of its largest reservoir, Lake Mead.

Climate scientists say the basin’s changing climate is leading to aridification, which refers to a long-term, increasing dryness rather than a seasonal variation.

So how is the Colorado River’s water divided?

Colorado River water is allocated using more than a century’s worth of legal deals, court decisions, contracts and government regulations, collectively called “the law of the river.” Its cornerstone? The Colorado River Compact of 1922.

The Colorado River doesn’t flood, surge and shrink as it did in its natural state. For most of its length, the river’s flows are controlled by a system of reservoirs, dams, ditches, canals and tunnels. Federal and state officials use that infrastructure to share and deliver the river’s water based on the law of the river. 

The 1922 compact split the river’s estimated supply equally between sub-basins, the Upper and Lower Colorado River basins. Each sub-basin can put 7.5 million acre-feet of water to use each year.

These sub-basins were designated for administrative and political purposes. Upper Division states include Colorado, New Mexico, Wyoming and Utah. Lower Division states include Arizona, California and Nevada. This can get complicated since parts of Lower Division states are in the Upper Basin and vice versa.

From there, water rights are primarily allocated based on the idea of “first in time, first in right,” called the doctrine of prior appropriation. (This is included in Colorado’s 1876 Constitution.) That means in times of shortage, water gets cut off first for the most recently established water rights, referred to as junior rights.

What was left out of the Colorado River Compact?

The Colorado River Compact left out key factors, like evaporation from reservoir storage and water used by Native American tribes and Mexico. As a result, it overestimated the amount of water available for use.

Today, as climate change puts pressure on the river system, the annual flow is about 12 million acre-feet or less. But the 1922 compact’s creators — state and federal officials, and their teams of attorneys and experts — believed there was as much as 20 million acre-feet of water in the river each year.

They based their calculations on flawed estimates of the river’s supply and prioritized data from one of the wettest periods in the past 1,300 years. They also ignored available research that said the supply was closer to 15 million acre-feet — a choice in favor of political expediency, some experts say.

In 1944, the U.S. established the Mexican Water Treaty with Mexico, which was not included in the 1922 compact negotiations. The treaty allocated 1.5 million acre-feet to Mexico.

Tribal governments, and thus water usage by tribes, were also not represented in the negotiations. There is only one line in the compact that acknowledges tribes at all: “Nothing in this compact shall be construed as affecting the obligations of the United States of America to Indian tribes.”

How much water does Colorado get from the Colorado River?

It varies. Each year, Colorado can use up to 51.75% of the river’s available water supply for the Upper Basin states. If the basin’s full allocation of 7.5 million acre-feet of water was put to use, Colorado would receive about 3.85 million acre-feet of water.

But, the Upper Basin states have never used their full allocation. When there is a full supply for all irrigators, the Upper Basin could use slightly more than 4.5 million acre-feet in a year. In 2021, the estimate was closer to 3.5 million acre-feet because of drought’s impact on the water supply. 

The Upper Basin’s water supply is distributed among states according to the 1948 Upper Colorado River Basin Compact, which allocates 11.25% to New Mexico, 23% to Utah and 14% to Wyoming. Because a portion of Arizona is within the Upper Basin, it receives 50,000 acre-feet per year from the Upper Basin’s allotment.

This percentage method was key to the compact: Most of the Colorado River Basin’s water supply falls in the Upper Basin, but that supply fluctuates from year to year. As Colorado’s water officials like to emphasize, that means the Upper Basin water users are forced to adapt to the amount of available water in any given year. 

When there are multiple years of drought in a row in Colorado, reservoirs can fall to record lows; farmers might see their water cut off early, depending on their water rights; and low water years can impact aquatic ecosystems and recreation economies.

How much Colorado River water goes to Arizona, California and Nevada? 

The Lower Basin’s 7.5 million acre-feet is split up so that Arizona can use up to 2.8 million acre-feet of Colorado River water; California, 4.4 million acre-feet; and Nevada, 300,000 acre-feet. 

This setup was established by the Boulder Canyon Project Act of 1928, which also ratified the 1922 Colorado River Compact, and a 1963 U.S. Supreme Court case between Arizona and California.

Lower Basin states are downstream from two massive reservoirs, Lake Mead and Lake Powell, which help insulate water users from the river system’s highs and lows. The reservoirs can store up to 53.9 million acre-feet of water, or about 92% of the river basin’s entire storage capacity.

How much water do tribes get from the Colorado River Basin?

Upper Basin tribes had federally recognized rights to 1.14 million acre-feet per year, and Lower Basin tribes had recognized rights to 2.06 million acre-feet per year, as of 2021. That equals about 26% of the Colorado River’s average flow of 12.44 million acre-feet per year between 2000 and 2018.

But that’s not the whole picture.

Tribes have some of the most senior water rights in the basin, but about a dozen still had unquantified rights as of 2021. That means the overall share of tribal water in the Colorado River Basin could change as legal quantification processes wrap up, experts say.

Tribes may also have quantified water rights but lack the infrastructure to deliver water to homes, businesses and farms on tribal lands. This is the case for both tribes that have reservation land in Colorado, the Ute Mountain Ute and Southern Ute tribes.

Tribal water has been a source of contention in the basin. Other water users are worried their more junior water rights will be cut off. Or, nontribal users are accustomed to tapping into the unused tribal water that flows downstream each year — a practice that Colorado’s top river official said can’t continue

Tribes say they have a right to use their water or at least be compensated for its use by others. They’re also working to be included in Colorado River negotiations, from which they’ve historically been excluded.

Is the Colorado River overused?

Lower and Upper Basin states might have slightly different answers to this question for political reasons, but in general, yes — especially as the river’s supply continues to shrink.

Many of the Colorado River Basin’s waterways are overallocated, which means that in dry years, some users have their water cut off early.

Colorado water officials often argue that Lower Basin states use more than their 7.5 million acre-feet share of water. The Colorado Water Conservation Board estimates that the Lower Basin and Mexico used 9.4 million acre-feet to 10 million acre-feet each year between 2019 and 2021, when the entire basin was in particularly intense years of drought. Part of the issue, Colorado officials say, is that the Lower Basin does not account for losses due to evaporation and leaky infrastructure.

The intensity of the drought over the past two decades is also putting new pressures on the rules that guide how water is shared. State and federal officials have been trying to adapt these guidelines to the river’s shrinking supply, but the supply/demand gap has not been resolved.


What is the plan for the future?

Federal, state and tribal officials are still figuring that out. In June, officials launched a new round of negotiations focused on how to share the river’s water in times of shortage over the long term. They must agree on a plan by 2026, when the current rules expire.

These negotiations, along with more near-term planning through 2026, focus on the rules for water storage and releases from Lake Mead and Lake Powell, the basin’s two largest water storage reservoirs.

The Colorado River Basin states are not considering changes to the 1922 Colorado River Compact and the 1948 Upper Colorado River Basin Compact as part of these negotiations, according to the Colorado Water Conservation Board.

Attorneys are spinning legal arguments and quibbling over verbs to strengthen each state’s position in the negotiations. One of the biggest legal sticking points, according to water attorneys not involved in the negotiations, is one phrase in the 1922 compact: Upper Basin states “will not cause the flow of the river at Lee Ferry to be depleted” below 75 million acre-feet for any period of 10 consecutive years.

Lee Ferry, downstream from Lake Powell in Arizona, is the dividing point between the Upper and Lower Basins — not to be confused with nearby Lees Ferry, the historic river crossing and boating access point.

Lower Basin entities may see this phrase as a delivery obligation, while Upper Basin states may pursue the argument that they are not “causing” the depletions, but rather that climate change and Lower Basin overuse are doing that. The debate seeks to inform how water would be curtailed if the river reaches more extreme shortages than have existed thus far.

Shannon Mullane writes about Western water issues for The Colorado Sun and her work is funded by a grant from the Catena Foundation. She focuses on the Colorado River Basin, tribal affairs related to water, and West Slope water issues.
Born in East Tennessee, Shannon has been in Colorado for a decade or so and is holding down the fort in Durango, the Sun's latest outpost. Before joining the Sun's team, she contributed award-winning reporting on government, environment, health and more as a staff writer for The Durango Herald and as an intern for the Colorado Independent. She also earned a master's in journalism from the University of Colorado Boulder. Shannon is conversational in French, trying really hard in Spanish, and often spotted baking or enjoying live music.