Park visitors look at a bleached 'bathtub ring' that is visible on the banks of Lake Mead near the Hoover Dam.

Visitors to Lake Mead, a reservoir in Nevada and Arizona that is fed by the Colorado River, see a bleached bathtub-like ring on its banks, indicating past water levels.Credit: Justin Sullivan/Getty

Seven US states this week finally agreed on a plan to cut their use of water from the Colorado River, an icon in the nation’s Southwest that supplies water to more than 40 million people, including in Mexico. The river, battered by overuse, drought and climate change, has been drying up. Although scientists welcome the plan, they say the agreement is only a temporary fix to a much thornier problem.

“It does not change the fundamental problem of the overallocation of the Colorado River,” says Kathryn Sorensen, an economist at Arizona State University in Phoenix. “But it helps.” Combined with an unexpectedly wet winter in the western United States, which is now filling the river with snowmelt, the agreement buys time for officials to negotiate a more sustainable solution.

From its headwaters in the Rocky Mountains to where it empties into the Gulf of California, the 2,300-kilometre-long river serves farmers, homeowners, businesses and many others, including people in 30 tribal nations and in cities including Los Angeles. The 22 May announcement is the latest in a decades-long dispute over who should cut back on their water use to preserve the river. The ‘lower basin’ states of California, Arizona and Nevada — the three that are farthest downstream as Colorado flows to the sea, with California the heaviest user of the river’s water — have agreed to conserve 3 million acre-feet (3.7 trillion litres) of water between now and 2026. That amount is enough to supply around 6 million households for a year.

DRYING UP. Graphic shows the decline in water levels at Lake Mead owing to overuse, drought and climate change since 2000.

Source: US Bureau of Reclamation

Researchers say that the Colorado River is overtaxed by serving more people than it can handle, in a part of the country that doesn’t get a lot of rainfall. A drought that began in 2000 has caused the river’s water levels to drop steadily — culminating last year when the massive Lake Mead reservoir in Nevada and Arizona shrank (see ‘Drying up’) to the point that long-lost human bodies emerged on the shoreline. If the levels in Lake Mead and in Lake Powell in Arizona and Utah — both fed by the river — drop low enough, water will no longer pass through their dams to generate the thousands of megawatts of hydropower that are used by people across the west. Both reservoirs are currently only about 30% full, down from 95% full in 2000.

The inevitability of climate change

One bright spot in the Colorado River outlook is this winter’s heavy snows, which are now melting from the mountains and feeding the river. As a result, lakes Powell and Mead are starting to rise faster than they have in recent years. But scientists say that it is a rare event, and not something to be counted on in the future. “This one year doesn’t get us out of it,” says Jack Schmidt, a geomorphologist at Utah State University in Logan.

Climate change, however, is plodding and relentless — and it’s expected to increase the frequency of droughts in the Colorado River basin. Every temperature increase of 1 ºC in the upper part of the river basin leads to a 9.3% drop in the river’s flow, the US Geological Survey has estimated1. Long-term strategies are needed to account for the inevitability of climate change, as well as the uncertainty of wet and dry seasons, says Kevin Wheeler, a water resource engineer at the University of Oxford, UK.

Reducing water use is a linchpin to all such plans. In California and Arizona, for instance, water management officials are ramping up plans to recycle wastewater into drinking water. Some are experimenting with encouraging changes in residents’ behaviour by, for instance, charging more for water in the summer than in the winter. Other efforts focus on cutting back on agricultural uses of water — which is key, because farmers account for 75–80% of the water consumption from the Colorado River, much of which is used to grow hay and other fodder for livestock. Technologies such as drip irrigation, which delivers water directly to plants rather than flooding an entire field, can help — although their effectiveness is debated.

A temporary fix

The four states that are upstream along the river — New Mexico, Utah, Colorado and Wyoming — have agreed with the lower-basin states. Together, they are asking the federal government, which oversees water releases from lakes Powell and Mead, to consider the new plan as a way forward between now and 2026. The US Department of the Interior has the power to modify, reject or accept the plan. The proposed cutback is only around half of what the department had been asking of the group of seven, but it still agreed to pay US$1.2 billion to support the proposal, for instance by paying farmers to let their fields lay fallow.

That’s a lot of money to support what is only a temporary fix, says Jennifer Gimbel, a water policy expert at Colorado State University in Fort Collins. “It concerns me that we’re spending that kind of money when we should be focused on a more permanent solution,” she says. Any permanent solution will involve a complex stew of government-backed incentives to conserve water across the entire river basin, all trying to balance the needs of many users.

The agreement pushes off any further decision-making until 2026, when interim water-management guidelines are due to expire. Between now and then, the United States, Mexico and tribal nations must come up with an approach for how they will conserve the river’s limited resources in future.

There’s no time to waste, Sorensen says. “Everyone understands that time is of the essence.”