Managing Colorado River risk.

In the 1920s, E. C. LaRue, a hydrologist at the United States Geological Survey, did an analysis of the Colorado River Basin that revealed the river could not reliably meet future water demands. No one heeded his warning. One hundred years later, water flow through the Colorado River is down by 20% and the basin's Lake Powell and Lake Mead—the nation's two largest reservoirs—are projected to be only 29% full by 2023. This river system, upon which 40 million North Americans in the United States and Mexico depend, is in trouble. But there is an opportunity to manage this crisis. Water allocation agreements from 2007 and 2019, designed to deal with a shrinking river, will be renegotiated over the next 4 years. Will decision-makers and politicians follow the science? It has been said that climate change is water change. Globally, the effects on rivers vary widely, from increased risk of flooding in some places, to short-run increases in river flows in others as glaciers melt and catastrophes ensue once the glaciers are gone. The only constant is change, and our inability to rely on the way rivers used to flow. Like many snowmelt-fed rivers, for the Colorado this translates into less water for cities, farms, and the environment. Research published over the past 5 years makes the threat clear. Run-off efficiency—the percentage of rain and snow that ends up as river water—is down, with half the decline since 2000 attributed to greenhouse-driven warming. For every 1°C of warming, researchers expect another 9% decline in the Colorado's flow. This year's snowpack was 80% of average but is delivering less than 30% of average river flows. Hot, dry summers bake soils, reducing flows the following year. The Colorado is not unusual. Researchers have identified similar patterns in other North American rivers, as well as in Europe, Asia, Africa, and Australia. Colorado River water management has a long and uneasy relationship with science. LaRue's analysis of the early 20th century was brushed aside in favor of larger, more aspirational estimates of the river's flow made by bureaucrats who wanted to build dams. Scientists who agreed with LaRue—there were many—were ignored. This left the river overallocated and put the basin at risk. Fortunately, there has since been progress in forging water management plans on the basis of science. For example, the US Bureau of Reclamation has been incorporating climate change into its analyses for more than a decade. Admirably, it overcame some of the political and technical challenges of incorporating the effects of climate change in the water allocation rules adopted in 2019. Models used to support decision-making were adapted to incorporate the 21st-century's declining flows. Computer simulations showing emptying reservoirs were enough to convince decision-makers of the need to cut back. But have the modelers gone far enough? The scientific challenges are formidable. Although the direction of change—a shift toward less river water—is clear, the details can be murky. This is a challenge for the handoff from science to the world of policy and politics. But we cannot allow that murkiness to stand in the way of taking seriously what the climate science is telling us. As the basin's water management community prepares for a new round of negotiations over the water allocation rules, how bad of a “worst case scenario” should be considered and who will get less water as a result? It is tempting to use today's 20% flow decline as the new baseline—that is, modeling future reductions on the basis of what has already been observed. But only by planning for even greater declines can we manage the real economic, social, and environmental risks of running low on a critical resource upon which 40 million North Americans depend. The United States and Mexico—not just America's West and Southwest—can't afford to get this wrong. There are still political challenges that harken back to the struggles of E. C. LaRue a century ago—namely, as political boosters chose overoptimistic estimates of the river's flows to make their jobs easier. Climate science indicates that there will likely be less water in the Colorado River than many had hoped. This is inconvenient for 21st-century decision-makers, and overcoming their resistance may be the hardest challenge of all.