Colorado River Drainage Basin Explained

Life in the southwestern U.S. as we know it exists thanks to the water of the Colorado River, which flows for approximately 1,450 miles from the Rockies to the Gulf of California.

The river gets its water from the Colorado River drainage basin, which spreads some 246,000 square miles.

A drainage basin is an area where all precipitation flows to the same river, or set of streams.

The Colorado River basin is made up of all of Arizona, parts of California, New Mexico, Colorado, Utah, Nevada, and Wyoming, and two Mexican states—Baja California and Sonora—although the final two states contribute little runoff to the river.

For management purposes, the basin is divided into the Upper Basin—New Mexico, Colorado, Utah, and Wyoming—and the Lower Basin—California, Nevada and Arizona.

Map of Colorado River basin
A photo shows the Colorado River basin. Newsweek explains exactly what a drainage basin is. USGS

Elizabeth Koebele, associate professor of political science at the University of Nevada, Reno, told Newsweek: "The Colorado River provides water for more than 40 million people in cities from Denver, Colorado, to Phoenix, Arizona; over 5 million acres of highly productive agricultural land; important industries, including outdoor recreation; and numerous species and critical habitats. It is so important because it is the major water source for this highly arid region—we simply would not be able to have the society we have in the western U.S. without the Colorado River."

The basin gets its water from snowpack, which runs down from the northern Rocky Mountains. This runoff is then captured in the Colorado River. Before the 1930s, the Colorado River flowed wild and untamed. It often flooded agricultural lands and caused huge problems for those living nearby.

However, after the construction of the Hoover Dam—which lies on the Nevada-Arizona border, forming Lake Mead on the Colorado River—in the 1930s, the water could finally be managed. There are now several dam systems along the river that irrigate the basin states.

The snowpack running down from the basin is now captured in man-made reservoirs such as Lake Powell and Lake Mead. In these reservoirs, the water is also used to generate hydropower for the basin states.

Before the construction of some of the dams on the Colorado River—such as the Hoover Dam—farmers downstream only had a lot of water in the spring when the snowpack melted. The water would then peter out by the end of the summer. However, with the construction of the dam, the flow could be controlled, meaning farmers had access to water 365 days a year.

Douglas Kenney, director of the Western Water Policy Program at the University of Colorado Law School in the Getches-Wilkinson Center for Natural Resources, Energy and the Environment, told Newsweek that with the Colorado River, it is useful to keep three different geographic areas in mind.

Colorado river and grand canyon
A stock photo shows the Colorado River running through the Grand Canyon. The Colorado River is fed water through the 7 basin states. MasterLu/Getty

"The basin is the hydrologic basin, meaning those areas where a drop of precipitation theoretically would drain eventually to the Colorado River itself. The 'service area' is those areas that receive Colorado River water via the infrastructure that has been built," Kenney said.

"In the Colorado, a lot of users are in the service area but not the basin, with some obvious examples being the Southern California urban areas (L.A. and San Diego) and Colorado's Front Range. This is a large population, at least half the total number of people using the river's water, but for most of these users the Colorado River is just one of several water resources, so how important the Colorado is in any given year is largely dependent on how 'healthy' their other supplies are."

The snowpack in the Sierras, for example, is very high this year. Much of this water will get pumped to L.A. This means that the Colorado River flows to the city are less important than they would have been last year, when the Sierras experienced a very dry year.

"The third area of note is the state lines of the 7 basin states. Each state can use their apportionment anywhere within their state lines, which is why we have this phenomenon of service areas," Kenney said. "In some places, namely the Great Lakes, the basin states there face a lot of restrictions if they try to use water in a part of their state that is outside of the hydrologic basin. It applied to the arid West, a policy like that would dramatically change the location (or existence) of many major cities."

From this, it is clear why the Colorado River is so important to life in the southwest.

But in 2023 the basin states are facing a barrage of issues. The southwest is in the grips of a 23-year megadrought, which is not showing signs of lifting anytime soon.

This has made weather patterns that create seasonal snowpack less frequent and harder to predict. This means the Colorado River, and its reservoirs, are drying up and not replenishing quickly enough.

"There are two key issues at hand in the Colorado River Basin. The first is that the basin is overallocated, meaning we have given people legal rights to use more water than actually exists in most years. This has caused the gradual overdraw of water stored in reservoirs. Second, the basin has been facing a megadrought since 2000, causing streamflow to decline," Koebele said.

"Higher temperatures and a trend toward long-term aridification of the region are causing greater evapotranspiration and reducing 'runoff efficiency,' or the amount of water that actually runs off to the stream from a given amount of snow."

Koebele said that even during good years, when basin states get the same amount of snow as in the past, it still translates to less streamflow.

"These climatic changes are exacerbating the problems caused by historical overallocation. Together, these problems have led to the lowest reservoir levels ever and water shortages throughout the basin," Koebele said.

Lake Mead, the most famous of the Colorado River reservoirs, is just below 30 percent of its total capacity. In July 2022, its water levels hit the lowest they have ever been, at 1,040 feet. It is feared that if it keeps declining, it could hit dead pool levels at 895 feet. This is the level in which water would no longer flow past the Hoover Dam, and operations would cease.

This is not all though. Kenney said that the basin is also experiencing a huge threat from wildfires.

"Just as climate change is drying out the river, it is also drying out and killing many of the forested areas where the river originates, primarily on the west slope of the Rocky Mountains," Kenney said.

"Healthy forests help ensure relatively constant river flows; unhealthy forests, however, tend to burn, and then precipitation runs off of those lands in muddy, violent torrents that cause all sorts of problems for water managers and others."

In the basin, there is also the issue of the health of agricultural communities.

"Most of the really profitable agriculture is outside the basin—in the service area—in southern California. Inside the basin, there's a lot of low value (in an economic sense) agriculture that is increasingly vulnerable to criticism given the water shortages impacting other users. That's really an issue about values and equity, more so than science or technology," Kenney said.

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