In 1940, 630 beavers lived in the Kawuneeche Valley in Colorado. By 2023, only six remained. Willow trees had thinned, wetlands had disappeared, and roughly 90 percent of the valley’s surface water was gone. The valley sits at the headwaters of the Colorado River, which supplies drinking water to about 40 million Americans along its 1,450-mile path through the western United States. Ecologists had…