Where does the river on which the United States' largest reservoir is located originate?
Colorado River:  Starting in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border. Lake Mead:  Lake Mead is on the Colorado River, about 24 mi from the Las Vegas Strip, southeast of the city of Las Vegas, Nevada, in the states of Nevada and Arizona.
the central Rocky Mountains