Geologists have solved the mystery of the disappearance from the geological record, millions of years ago, of one of North America's most important waterways: the Colorado River. A paper published in Science shows that the river flowed into an upstream lake over the course of a few million years, then likely flowed for the first time into the Grand Canyon. The moment marked the Colorado River's transition to a continental-scale river as it made its way down to the Gulf of California.