Enlarge / The Hemenway Harbor Marina at Lake Mead, the country’s largest man-made water reservoir, formed by Hoover Dam on the Colorado River in the Southwestern United States, as viewed from Boulder Beach on August 14, 2023. The Lake Mead, a national recreation area, located within the states of Nevada and Arizona 24 miles east of the Las Vegas Strip, serves water to the states of Arizona, California, Utah, Colorado, and Nevada, as well as parts of Mexico, providing fresh water to nearly 20 million people and large swaths of farmland. (credit: George Rose/Getty Images)

It’s been one of the wettest years in California since records began. From October 2022 to March 2023, the state was blasted by 31 atmospheric rivers—colossal bands of water vapor that form above the Pacific and become firehoses when they reach the West Coast. What surprised climate scientists wasn’t the number of storms, but their strength and rat-a-tat frequency. The downpours shocked a water system that had just experienced the driest three years in recorded state history, causing floods, mass evacuations, and at least 22 deaths.

Swinging between wet and dry extremes is typical for California, but last winter’s rain, potentially intensified by climate change, was almost unmanageable. Add to that the arrival of El Niño, and more extreme weather looks likely for the state. This is going to make life very difficult for the dam operators tasked with capturing and controlling much of the state’s water.

Like most of the world’s 58,700 large dams, those in California were built for yesterday’s more stable climate patterns. But as climate change taxes the world’s water systems—affecting rainfall, snowmelt, and evaporation—it’s getting tough to predict how much water gets to a dam, and when. Dams are increasingly either water-starved, unable to maintain supplies of power and water for their communities, or overwhelmed and forced to release more water than desired—risking flooding downstream.