Colorado River Raft Trip in Westwater canyon on the Colorado river
The Colorado begins at La Poudre Pass in the Southern Rocky Mountains of Colorado, at just under 2 miles (3 km) above sea level.[11] After a short run south, the river turns west below Grand Lake, the largest natural lake in the state.[12] 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 the portion of the state west of the Continental Divide. As it flows southwest, it gains strength from many small tributaries, as well as larger ones including the Blue, Eagle and Roaring Fork rivers. After passing through De Beque Canyon, the Colorado emerges from the Rockies into the Grand Valley, a major farming and ranching region where it meets one of its largest tributaries, the Gunnison River, at Grand Junction. Most of the upper river is a swift whitewater stream ranging from 200 to 500 feet (60 to 150 m) wide, the depth ranging from 6 to 30 feet (2 to 9 m), with a few notable exceptions, such as the Blackrocks reach where the river is nearly 100 feet (30 m) deep.[13][14] In a few areas, such as the marshy Kawuneeche Valley near the headwaters[15] and the Grand Valley, it exhibits braided characteristics.[14]
Arcing northwest, the Colorado begins to cut across the eponymous Colorado Plateau, a vast area of high desert centered at the Four Corners of the southwestern United States. Here, the climate becomes significantly drier than that in the Rocky Mountains, and the river becomes entrenched in progressively deeper gorges of bare rock, beginning with Ruby Canyon and then Westwater Canyon as it enters Utah, now once again heading southwest.[16] Farther downstream it receives the Dolores River and defines the southern border of Arches National Park, before passing Moab and flowing through The Portal, where it exits the Moab Valley between a pair of 1,000-foot (300 m) sandstone cliffs.[17]
In Utah, the Colorado flows primarily through the slickrock country, which is characterized by its narrow canyons and unique folds created by the tilting of sedimentary rock layers along faults. This is one of the most inaccessible regions of the continental United States.[18][19] Below the confluence with the Green River, its largest tributary, in Canyonlands National Park, the Colorado enters Cataract Canyon, named for its dangerous rapids,[20] and then Glen Canyon, known for its arches and erosion-sculpted Navajo sandstone formations.[21] Here, the San Juan River, carrying runoff from the southern slope of Colorado's San Juan Mountains, joins the Colorado from the east. The Colorado then enters northern Arizona, where since the 1960s Glen Canyon Dam near Page has flooded the Glen Canyon reach of the river, forming Lake Powell for water supply and hydroelectricity generation
In Arizona, the river passes Lee's Ferry, an important crossing for early explorers and settlers and since the early 20th century the principal point where Colorado River flows are measured for apportionment to the seven U.S. and two Mexican states in the basin.[24] Downstream, the river enters Marble Canyon, the beginning of the Grand Canyon, passing under the Navajo Bridges on a now southward course. Below the confluence with the Little Colorado River, the river swings west into Granite Gorge, the most dramatic portion of the Grand Canyon, where the river cuts up to one mile (1.6 km) into the Colorado Plateau, exposing some of the oldest visible rocks on Earth, dating as long ago as 2 billion years.[25] The 277 miles (446 km) of the river that flow through the Grand Canyon are largely encompassed by Grand Canyon National Park and are known for their difficult whitewater, separated by pools that reach up to 110 feet (34 m) in depth.