The Green River doesn’t make a lot of sense at first glance. The Colorado River’s largest tributary flows through a nearly 2,300-foot-deep canyon inside of northeastern Utah’s Uinta mountain range. This makes the story of how the Green and Colorado Rivers met so perplexing to geologists like Adam Smith at Scotland’s University of Glasgow. “The merging of the Green and Colorado Rivers millions of years ago altered the continental divide of North America,” he explained in a statement. This timeline corresponds to previous work suggesting the Green River carved into the mountains and joined with the larger Colorado system.