By now, you may have heard that the Colorado River is drying up.
The river’s flow is down by about 20 percent, compared to the 1900s, and the two largest reservoirs it feeds are less than a third full. The water in Lake Mead, the nation’s biggest reservoir, has dropped more than 150 feet in the last two decades, leaving little water for the more than 40 million people who depend on the river.
Part of the reason why the Colorado River is shrinking is the dwindling amount of snow and rain. The West is in its 23rd year of drought, which research suggests could be the driest period in the last 1,200 years, made worse by climate change.
Then there is the sheer number of cities and farms that are sucking down water. About three-quarters of all water that humans consume from the Colorado River goes toward irrigating farms, which, among other things, supply nearly all of the nation’s winter veggies.
But a key reason why the Colorado River is running out of water has more to do with math than anything — bad math.
One hundred years ago, government officials divvied up water in the Colorado River among the seven states that rely on it including Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. The agreement, known as the Colorado River Compact, was based on one critically important number: the total amount of water that the Colorado River can supply yearly.
Ignoring the best science of the time, officials claimed the river could provide about 20 million acre-feet per year (an acre-foot is the amount of water needed to fill an acre with one foot of water), according to the 2021 book Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River. That number was way too high, the authors write, meaning that officials promised states water that simply didn’t exist.
“They had conjured up a larger Colorado River than nature could actually provide,” wrote authors Eric Kuhn, a retired water official, and John Fleck, a writer and former director of the University of New Mexico’s Water Resources Program. “The twenty-first century’s problems on the river are the inevitable result of critical decisions made by water managers and politicians who ignored the science available at the time.”