Colorado River - Wikipedia The Colorado begins at La Poudre Pass in the Never Summer Mountains in Rocky Mountain National Park, 10,184 ft (3,104 m) above sea level. After a short run south, the river turns west below Grand Lake, the largest natural lake in the state. For the first 250 miles (400 km) of its course, the Colorado carves its way through the mountainous Western Slope, a sparsely populated region defined by ...