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The Colorado River is a river in the Southwestern United States and northwestern Mexico, approximately 1,450 miles (2,330 km) long, draining a part of the arid regions on the western slope of the Rocky Mountains. The natural course of the river flows from 25 km (16 mi) north of Grand Lake, Colorado into the Gulf of California between the Baja California peninsula and mainland Mexico.
The Colorado River's headwater is located in Rocky Mountain National Park about 25 mi. north of Lake Granby at the northern tip of Colorado's Grand County where Jackson County and Larimer County intersect. This is the unique geologic point where the Continental Divide intersects the Colorado River drainage basin to the west and the eastern flowing drainage basins for Jackson County's N. Platte River and the Larimer County headwaters of the Laramie River, Cache La Poudre River and the northernmost main branch of the South Platte River. Below Rocky Mountain National Park, the river flows through the Kawuneeche Valley, also part of the Park, into Grand Lake, Colorado's largest and deepest natural lake. By law it can fluctuate no more than one vertical foot, so the Colorado River actually flows into Shadow Mountain Reservoir where it encounters the first of many dams in its journey to the Gulf of California. The physical connection between Shadow Mountain Reservoir and Grand Lake is not the course of the Colorado River, but a logistical piece of a larger trans-basin water storage and delivery project. This is the Colorado-Big Thompson Project that diverts the headwaters of the Colorado River to Colorado's Front Range and Eastern Plains on the other side of the Continental Divide. The next stop on the river's journey, Lake Granby, is used as a reservoir in this same project. Windy Gap Reservoir at the confluence of the Fraser and Colorado Rivers, west of the town of Granby, is another. From there, U.S. Highway 40 roughly parallels the river to the town of Kremmling, where the Blue River joins, before it enters Gore Canyon, to the west.
Most of the river's uppermost tributaries within Colorado are small. There are exceptions, such as the Gunnison River and Roaring Fork River, in which massive amounts of water flow. About a hundred miles later it meets the Eagle River in the town of Dotsero, Colorado, and where I-70 parallels the river through Glenwood Canyon. The river then passes through the city of Glenwood Springs where it is joined by the swift flowing Roaring Fork River. West of Glenwood Springs, the Colorado runs through the Grand Valley and is joined by the Gunnison River at Grand Junction. From there it flows in an arc west-north to the Utah border and Westwater Canyon. Here, the Colorado ranges from 200 to 1,200 ft (61 to 370 m) wide and from 6 to 30 ft (1.8 to 9.1 m) in depth with occasional deeper
The river turns southwest near Fruita, Colorado, and is joined by the Dolores River soon after entering Utah. It partially forms the southern border of Arches National Park near Moab, Utah, and then passes by Dead Horse Point State Park and through Canyonlands National Park where it is met by one of its primary tributaries, the Green River. The Colorado River then flows into Lake Powell, formed by the Glen Canyon Dam, where the San Juan River joins. Below the dam, water released from the bottom of Lake Powell makes the river clear, clean, and cold. Just south of the town of Page, Arizona, the river forms the dramatic Horseshoe Bend, then at Lees Ferry is joined by another tributary, the warm, shallow, muddy Paria River, and begins its course through Marble Canyon. Here, the Colorado River ranges from 175 to 700 ft (53 to 210 m) in width and 9 to 130 ft (2.7 to 40 m) in depth.
At the southern end of Marble Canyon, the river is joined by another tributary, the Little Colorado River, and the river then turns abruptly west directly across the folds and fault line of the plateau, through the Grand Canyon and Grand Canyon National Park, which is 349 km long (217 miles) and from 6 to 30 km (4 to 20 miles) between the upper cliffs. The walls, 4,000 to 6,000 ft (1,200 to 1,800 m) high, drop in successive escarpments of 500 to 1,600 ft (150 to 490 m), banded in splendid colors toward the narrow gorge of the present river.
Below the confluence of the Virgin River in Nevada, the Colorado River abruptly turns southward. Hoover Dam, built during the Great Depression, forms Lake Mead, a popular recreation site as well as the supplier of most of the water for the Las Vegas metropolitan area. From Hoover Dam, the river flows south and forms part of the boundary between Arizona and Nevada and between Arizona and California. Along the California-Arizona reach of the river, four additional dams are operated to divert water for municipal supplies and agricultural irrigation, and for recreational uses. Lake Mohave, formed by Davis Dam, lies in the southern portion of the Lake Mead National Recreation Area. Lake Havasu, formed by Parker Dam, provides recreation as well as the home of the retired U.K. London Bridge, now the New London Bridge. The two remaining dams supply irrigation water: Palo Verde Diversion Dam and Imperial Dam. Here, the Colorado River ranges in width from 700 to 2,500 ft (210 to 760 m) and from 8 to 100 ft (2.4 to 30 m) in dept
Below the Black Canyon the river lessens in gradient and in its lower course flows through the Colorado Desert in a broad sedimentary valley's distinct estuarine plain upriver from Yuma, where it is joined by the Gila River. The channel through much of this region is bedded in a dike-like embankment lying above the floodplain over which the escaping water spills in time of flood. This dike cuts off the flow of the river to the remarkable low area in southern California known as the Salton Sink in the Coachella Valley, and the Imperial Valley. The Salton Sink and its Salton Sea are located below sea level; therefore, the descent from the river near Yuma is very much greater than the descent from Yuma to the Gulf of California.
The lower course of the river—which forms the border between the Mexican state of Sonora on the mainland and the state of Baja California on the Baja California peninsula—is a dry or small stream most of the year due to the river being diverted into the Imperial Valley's irrigation source, the All-American Canal. Several miles from its mouth, the Hardy River enters, adding a little water to the often dry Colorado before it reaches the Gulf of California.