Colorado River begins at Granby, CO, just south of Rocky Mountain Nat Park and goes west towards Steamboat Springs, been there. What about Black Canyon of the Gunnison?
There may still be some old-timers who dispute that. What is now the Colorado River in Colorado used to be called the Grand River, and the name is reflected in nearby place names such as Grand Junction and Grand Mesa. The Colorado River flowed southwest from where the Grand River and the Green River met in Utah. Some relatives of mine who lived in Colorado refused to call the Grand River by its new name after it was changed in the 1920's.
In 1961, I rode in a 1954 Studebaker station wagon up a dirt road at the bottom of the Black Canyon of the Gunnison. The scenery was spectacular--sheer cliffs on either side of the Gunnison River, with its rapids. Today, that road is at the bottom of the Blue Mesa reservoir.
Yep, I rafted it last July. A nice canyon already exist by the time you get 35 nm miles from Granby to Kremmling.