Colorado River

[11] 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.

[16] 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.

[25] At the lower end of Grand Canyon, the Colorado widens into Lake Mead, the largest reservoir in the continental United States, formed by Hoover Dam on the border of Arizona and Nevada.

[26] Below the dam the river passes under the Mike O'Callaghan–Pat Tillman Memorial Bridge—which at nearly 900 feet (270 m) above the water is the highest concrete arch bridge in the Western Hemisphere[27]—and then turns due south towards Mexico, defining the Arizona–Nevada and Arizona–California borders.

Joseph C. Ives, who surveyed the lower river in 1861, wrote that "the shifting of the channel, the banks, the islands, the bars is so continual and rapid that a detailed description, derived from the experiences of one trip, would be found incorrect, not only during the subsequent year, but perhaps in the course of a week, or even a day.

[33] Below Imperial Dam, only a small portion of the Colorado River makes it beyond Yuma, Arizona, and the confluence with the intermittent Gila River—which carries runoff from western New Mexico and most of Arizona–before defining 24 miles (39 km) of the Mexico–United States border.

[36] Before 20th-century development dewatered the lower Colorado, a major tidal bore was present in the delta and estuary; the first historical record was made by the Croatian missionary in Spanish service Father Ferdinand Konščak on July 18, 1746.

[82] Upstream of the massive Hoover and Glen Canyon Dams, inflows to Lake Powell still experience distinct spring highs and winter lows, though overall volumes have been reduced due to Upper Basin water diversions.

[92] At Morelos Dam, the remaining flow is diverted to irrigate the Mexicali Valley, leaving the Colorado River Delta almost entirely dry except for small amounts of agricultural wastewater.

[105] Sediments carried from the plateau by the Colorado River created a vast delta made of more than 10,000 cubic miles (42,000 km3) of material that walled off the northernmost part of the gulf in approximately 1 million years.

[112] Small numbers of Paleo-Indians of the Clovis and Folsom cultures inhabited the Colorado Plateau as early as 10,000 BCE, with populations beginning to increase in the Desert Archaic period (6000 BCE–0 CE).

The Puebloan people built many multi-story pueblos or "great houses", and developed complex distribution systems to supply drinking and irrigation water in Chaco Canyon in northwestern New Mexico[118] and Mesa Verde in southwest Colorado.

[121] Puebloan and Hohokam settlements were abruptly abandoned in the 1400s CE, due both to over-exploitation of natural resources such as timber, and severe drought that made it impossible to maintain irrigation systems.

We are three-quarters of a mile in the depths of the earth, and the great river shrinks into insignificance, as it dashes its angry waves against the walls and cliffs, that rise to the world above; they are but puny ripples, and we but pigmies, running up and down the sands, or lost among the boulders.

[180] The Denver and Rio Grande Western Railroad (D&RGW) quickly expanded into this area to serve mining boomtowns, crossing the Rockies to the south via the Black Canyon of the Gunnison River.

[183][156]: 78  In 1879 a group of Mormon settlers made their way to southeastern Utah, blasting the precarious Hole in the Rock Trail to cross the Colorado River at Glen Canyon, subsequently establishing the community of Bluff.

The D&SL completed a rail line into the upper headwaters of the Colorado River and blasted the Moffat Tunnel under the Continental Divide, but ran out of money before even reaching Utah.

The Upper and Lower Basin were each allocated 7.5 million acre-feet (9.3 km3) of water per year, a figure believed to represent half of the river's annual flow at Lee's Ferry.

[220] The Colorado River delta region in Mexico became a favored place for Americans to invest in agriculture in the late 19th century when Mexican President Porfirio Díaz welcomed foreign capital to develop the country.

[225] It took seven attempts, more than $3 million, and two years for the railroad, the CDC, and the federal government to permanently block the breach and restore the river's original course – but not before part of the Imperial Valley was flooded under a 45-mile-long (72 km) lake, today's Salton Sea.

The canal's intake is located at Imperial Dam, 20 miles (32 km) above Yuma, Arizona, which diverts the majority of the Colorado's flow with only a small portion continuing to Mexico.

[272] In late 2010, Lake Mead was approaching the "drought trigger" elevation of 1,075 feet (328 m), at which water supplies to Arizona and Nevada would be reduced in accordance with the Colorado River Compact.

[286] On June 14, 2022, Bureau of Reclamation Commissioner Camille Touton told the Senate Committee on Energy and Natural resources that additional cuts of 2–4 million acre-feet were required to stabilize reservoir levels in 2023.

[287] When the states were unable to come to an agreement about how to share the proposed cuts, Reclamation began the legal steps to unilaterally reduce releases from Hoover and Glen Canyon Dams in 2023.

[288] As of December 2022 the lower basin states of Nevada, Arizona, and California had not agreed on how to reduce water use by the approximately 30% required to keep levels in lakes Mead and Powell from crashing.

[300] More than 1,600 species of plants grow in the Colorado River watershed, ranging from the creosote bush, saguaro cactus, and Joshua trees of the Sonoran and Mojave Deserts to the forests of the Rocky Mountains and other uplands, composed mainly of ponderosa pine, subalpine fir, Douglas-fir and Engelmann spruce.

Today's desiccated delta, at only a fraction of its former size, no longer provides suitable habitat, and populations of fish, shrimp and sea mammals in the gulf have seen a dramatic decline.

[320] Large dams such as Hoover and Glen Canyon typically release water from lower levels of their reservoirs, resulting in stable and relatively cold year-round temperatures in long reaches of the river.

[322] These flow patterns have also made the Colorado more dangerous to recreational boaters; people are more likely to die of hypothermia in the colder water, and the general lack of flooding allows rockslides to build up, making the river more difficult to navigate.

[326] This pulse reached the sea on May 16, 2014, marking the first time in 16 years that any water from the Colorado flowed into the ocean, and was hailed as "an experiment of historic political and ecological significance" and a landmark in U.S.–Mexican cooperation in conservation.

Headwaters of the Colorado River in Rocky Mountain National Park , Colorado
A narrow river flows through a narrow gorge flanked by high rocky bluffs
Colorado River in the Grand Canyon seen from Pima Point, near Hermit's Rest
Top-down view of green agricultural lands surrounded by desert
Satellite view of the Colorado River valley near Yuma, Arizona ; Interstate 8 runs from left to right just below center.
Colorado River as it exits the United States into Mexico beneath the San Luis Colorado-Colonia Miguel Aléman Bridge (September 2009)
View of a river winding through a series of narrow gorges that dramatically double back on each other
The San Juan River near Mexican Hat, Utah
View of a brownish river flowing between vegetated banks, with high bluffs rising in the background
The Green River at Mineral Bottom, just north of Canyonlands National Park
Map showing the Upper and Lower Colorado River Basin, and adjacent areas supplied by Colorado River water.
The Black Suspension Bridge crosses the Colorado River in the Grand Canyon at Phantom Ranch , Arizona.
The average annual discharge of the Colorado River has shown a slight but noticeable decreasing trend between 1895 and 2004.
Annual Colorado River discharge volumes at Lee's Ferry between 1895 and 2004
View showing hardened flows of dark volcanic rock descending over the side of a canyon
Remnants of basalt flows from the Uinkaret volcanic field are seen here descending into the Grand Canyon, where they dammed the Colorado over 10 times in the past 2 million years.
Black and white photograph of a Native American woman holding a child
Navajo woman and child, photographed by Ansel Adams , c. 1944
La conquista del Colorado (2017), by Augusto Ferrer-Dalmau , depicts Francisco Vázquez de Coronado 's 1540–1542 expedition. García López de Cárdenas can be seen overlooking the Grand Canyon .
Boats of John Wesley Powell 's second Colorado River expedition in Marble Canyon , 1872.
Route of Powell's first expedition, 1869.
Lithograph of Fort Yuma, c. 1875
Historic photograph of the cable ferry at Lee's Ferry, prior to construction of the Navajo Bridge.
Front view of a dam in a narrow canyon, with water shooting out of the gates
Hoover Dam releasing water in 1998
The Grand Ditch, one of the earliest water diversions of the Colorado River, is still in use today.
The Little Oso Diversion Dam diverts water from the Little Navajo River and transports it through a series of tunnels into the Rio Grande basin as part of the San Juan–Chama Project .
Hoover Dam under construction, 1934
The Imperial Dam (bottom right) diverts water into the All-American Canal (center) running towards Imperial Valley.
Glen Canyon Dam, the largest dam of the Colorado River Storage Project
The main canal of the Central Arizona Project , crossing the Sonoran Desert
View of a reservoir where the water level has dropped, showing white deposits on the surrounding mountains
Lake Mead in 2010, showing the "bathtub ring" left behind by low water levels
View of a wide river flowing through a forested area, with jagged mountains in the background
Heavily forested banks of the Colorado River near Topock, Arizona
Most of the Colorado River basin water used by humans is used to grow feed for livestock—more than four times the amount used for crops for direct human consumption. [ 306 ]
View of a narrow green river flowing between high, reddish-brown cliffs
The Colorado was named for the reddish color caused by its natural sediment loads, but damming the river has caused it to acquire a clear green hue as seen here in lower Glen Canyon.
View of two small boats in a river, with high cliffs rising immediately behind them
A rafting party on the Colorado River