Nearly 40 million people rely on the Colorado River for water, from Wyoming to northern Arizona and beyond.
That’s an estimate seen again and again in news reports and writing about the West’s most important waterway.
But the exact number of people who use water from the river is unknown, says Michael Cohen, a senior fellow at the Pacific Institute who’s studied the Colorado River for decades. The institute is an Oakland, California-based water policy think tank.
The number is important because it could equate to bargaining power for states and cities with bigger populations amid a 26-year megadrought, he says.
And the estimate of 40 million people strikes Cohen as a little high.
That’s because, out of the roughly 40 million people thought to be relying on Colorado River water, he says 7 million people in Southern California don’t have the plumbing to access it.
“It’s kind of ill defined, but it’s just kind of the conventional fallback,” says Cohen.
The 40 million figure popped up in a 2012 study by the U.S. Bureau of Reclamation analyzing water supply and demand in the Colorado River Basin.
The number appears to have been revised down in a 2015 report by reclamation on metropolitan areas’ use of Colorado River water in the basin.
That 2015 report states that the “total population served in … metropolitan areas is approximately 29 million, or more than 85 percent of the total population served with Colorado River water supply.”
Cohen used those numbers to figure out that reclamation was saying there’s a total of about 34 million people in the U.S. receiving water from the basin. Not 40.
But he says the report’s 34 million people estimate includes 18 million people in the service area of the Metropolitan Water District of Southern California, a subtotal that Cohen says is high by about 6 million.
“The Metropolitan continues to say 19 million people rely on Colorado River water [in the service area], even though their internal documents say that's not the case,” he tells KNAU. The real number is more likely around 12.3 million, according to the California water district’s documents.
After factoring in those differences, as well as population growth in Arizona and parts of Nevada, Cohen wrote in a blog post in 2023 that a more accurate estimate would be roughly 35 million people using water from the Colorado River Basin, not 40.
So, why does it matter?
“It's important because it's useful to determine how much money should be spent and where the money is going,” Cohen says. “We're often doing a calculus: Who's going to benefit from this? How many people are benefiting? How many people rely on it? And what are the impacts if the system crashes, and how many people have other alternative sources of water they can rely on?”
He commonly runs across variations of the estimate, too, like that “40 million people use Colorado River water, [or] 40 million people use water from the Colorado River Basin, [or] 40 million people in the United States use water [from the Colorado River].”
Some people interpret the phrase “Colorado River water” as being limited to the main stem of the river. Others think it applies to all water in the basin, including tributaries. Things get even trickier when factoring in rapidly growing communities in northern Mexico.
“The bigger the number, then the more political power, the more implications or impacts it would have if more people are cut back,” he says. “If we don't measure it, then how are we going to take action to stabilize the system?”
It’s not absolutely essential to know whether 39 million people rely on the river, versus 40 million, he explains, but it is useful to have accurate numbers, and a lot of times “people just repeat numbers.”
It’s not just the number of people who use the river’s water: Exact numbers around how much Colorado River water is used and how many acres of farmland are irrigated using it are also fuzzy.
On top of that, while there’s been progress in recent years in settling the water rights of some Native American tribal nations, some rights are still unrecognized or unquantified.
In his 2023 blog post, Cohen wrote that those unrecognized water rights “challenge … efforts to estimate the location of future demands and uses as tribes develop their water rights and/or lease them to other users, again leading to significant uncertainty for Basin decision-makers.”
