Where Glaciers Melt, the Rivers Run Red Where Glaciers Melt, the Rivers Run Red

As the glaciers of South America retreat, the supply of freshwater is dwindling and its quality is getting worse.

Dionisia Moreno, a 70-year-old Indigenous farmer, still remembers when Shallap River, nearly 13,000 feet up in the Cordillera Blanca, brought crystal clear water brimming with trout to her village, Jancu. “People and animals alike could drink the water without suffering,” she said. “Now the water is red. No one can drink it.”

At a glance the river looks like a casualty of mining pollution; Peru is a major producer of copper, silver and gold, and the waters near abandoned mines often run a shade of rust. But the culprit is climate change. The Cordillera Blanca mountain range harbors the world’s largest concentration of tropical glaciers, which are particularly sensitive to rising temperatures and are a major source of freshwater in Peru.

For thousands of years, the glaciers were replenished with ice in the winter. But they have shrunk by more than 40 percent since 1968, uncovering rocks that, when exposed to the elements, can trigger chemical reactions that leach toxic metals into the water and turn it acidic.

The process, known as acid rock drainage, “creates a cascade reaction that pollutes water sources,” said Raúl Loayza, a biologist at Peru’s Cayetano Heredia University who researches water quality in the Andes. “It’s a big problem and is getting worse and worse.”