The Colorado River is the principal river of the southwestern United States and northwest Mexico. The 1,450-mile (2,330 km) river drains an expansive, arid watershed that encompasses parts of seven U.S. and two Mexican states. Rising in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau before reaching Lake Mead on the Arizona–Nevada line, where it turns south towards the international border. After entering Mexico, the Colorado forms a large delta, emptying into the Gulf of California between Baja California and Sonora.
Known for its dramatic canyons and whitewater rapids, the Colorado is a vital source of water for agricultural and urban areas in the southwestern desert lands of North America. The river and its tributaries are controlled by an extensive system of dams, reservoirs and aqueducts, which furnish water for irrigation and municipal supplies of almost 40 million people both inside and outside the watershed. The Colorado's steep drop through its gorges is also utilized for the generation of significant hydroelectric power, and its major dams regulate peaking power demands in much of the Intermountain West. Since the mid-20th century, intensive water consumption has dewatered the lower 100 miles (161 km) of the river such that it no longer reaches the sea except in years of heavy runoff.
Nomadic hunter-gatherer peoples first populated the Colorado River basin at least eight thousand years ago. Between 1,000 and 2,000 years ago, native peoples began to form large, sedentary agriculture-based civilizations, which may have been some of the most sophisticated indigenous cultures in North America. A combination of climate change and poor land use practices led to the collapse of these societies, but other native groups remained, many up to the present day. Europeans first entered the Colorado River watershed in the 1500s, with explorers from Spain mapping and claiming the area, which later became part of Mexico with its independence from Spain in 1821. Early contact between foreigners and natives was largely limited to the fur trade in the headwaters and sporadic trade interactions along the lower river.
After the Colorado River basin became part of the U.S. in 1846, the river course was still largely unknown, and the whereabouts of its headwaters and mouth were still the subject of myths and speculation. A number of expeditions charted the Colorado in the mid-19th century, of which one was the first to run the rapids of the Grand Canyon, led by John Wesley Powell in 1869. American explorers collected valuable information that would later be used to investigate the feasibility of developing the river for navigation and water supply. Large scale settlement of the lower basin by whites began in the mid-to-late 1800s, with steamboats providing transportation and trade along the Colorado and Gila Rivers. Lesser amounts settled in the upper basin, which was also the scene of major gold strikes in the 1860s and 1870s.
Major engineering of the river basin began around the start of the 20th century, with many guidelines for development established in a series of treaties both domestic and international known as the "Law of the River". The U.S. federal government was the main driving force behind the construction of hydraulic engineering projects in the river system, although many state and local water agencies were also involved. Most of the major dams in the river basin were built between 1910 and 1970, with the system keystone, Hoover Dam, completed in 1935. Because of these developments, the Colorado River is now considered among the most controlled and litigated in the world, with every drop of its water fully allocated. High rates of water removal for irrigation and industry combined with declines in natural runoff due to climate change could lead to severe shortages by the mid-21st century, endangering power generation and water supply.
The Colorado rises at La Poudre Pass east of the Never Summer Mountains in the Colorado Rockies, about 60 miles (97 km) northwest of Denver. The river runs south before turning west below Grand Lake, the largest natural lake in the state. After passing Kremmling it cuts a series of narrow canyons, including Gore, Glenwood, and De Beque. The Colorado emerges from the mountains at the Grand Valley, where it is joined by the Gunnison River before arcing northwest into desert Utah. Carving its way southwest across the Colorado Plateau, the Colorado forms Cataract Canyon and other gorges and receives its principal tributary, the Green River, before flowing into Lake Powell, a reservoir formed by the Glen Canyon Dam in Arizona nearly 200 miles (320 km) downstream.
The Colorado passes Lee's Ferry in Arizona, the official dividing point of the Upper and Lower Colorado River Basins, before swinging south then west through the Grand Canyon. Below Lake Mead – the largest man-made lake in the U.S., formed by Hoover Dam at the junction of Arizona and Nevada – the river turns sharply south. As it enters the Lower Colorado River Valley the Colorado delineates much of the Arizona–Nevada border, the entirety of the Arizona–California border, and is impounded by a series of dams, including Imperial Dam, where most of its flow is diverted into the All-American Canal to irrigate the Imperial Valley in California. Below the confluence with the Gila River the Colorado forms a short stretch of the Mexico–United States border before passing entirely into Mexico. It empties into the Gulf of California via a large estuary, the Colorado River Delta, roughly 75 miles (121 km) south of Yuma, Arizona.
With its headwaters at 10,184 feet (3,104 m), the Colorado River loses nearly two miles in elevation by the time it reaches the Gulf. Most of the Colorado above Lake Mead is a swift-moving whitewater river, with the exception of the region around Grand Junction, Colorado, where it exhibits braided characteristics, and the marshy Kawuneeche Valley near the headwaters. The lower river between Hoover Dam and the international border is generally a slow-moving, meandering stream. Much of the upper Colorado ranges from 200 to 500 feet (61 to 150 m) wide, compared with 500 to 1,000 feet (150 to 300 m) for the lower river, with an average depth of 10 to 30 feet (3.0 to 9.1 m). Some parts of the river are as shallow as 2 to 8 feet (0.61 to 2.4 m) in the lower course in dewatered sections near Yuma, and one notable section in the Grand Canyon reaches up to 110 feet (34 m) in depth.
The river was characterized by sweeping meanders, sandbars and islands that were subject to frequent course changes before channelization of the lower Colorado in the 20th century. 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." The delta and estuary of the Colorado River were once also subjected to a major tidal bore that has almost disappeared with reductions in river flow and some dredging of the estuary channel. The first historical record of the tidal bore was that made by the Croatian missionary in Spanish service Father Ferdinand Konščak on 18 July 1746. During spring tide conditions, the tidal bore formed in the estuary about Montague Island in Baja California and propagated upstream. It was locally called El Burro or burro.
In its natural state, the Colorado River poured about 15.7 million acre-feet (18.63 km3) into the Gulf of California each year, amounting to an average flow rate of 21,700 cubic feet per second (610 m3/s).[n 2] However, its flow regime was not at all steady – indeed, "prior to the construction of federal dams and reservoirs, the Colorado was a river of extremes like no other in the United States." Once, the river reached peaks of more than 100,000 cubic feet per second (2,800 m3/s) in the summer and low flows of less than 2,500 cubic feet per second (71 m3/s) in the winter on an annual basis. At Topock, Arizona, about 300 miles (480 km) upstream from the Gulf, a maximum historical discharge of 384,000 cubic feet per second (10,900 m3/s) was recorded in 1884 and a minimum of 422 cubic feet per second (11.9 m3/s) was recorded in 1935. In contrast, the regulated discharge rates on the lower Colorado below Hoover Dam rarely exceed 35,000 cubic feet per second (990 m3/s) or drop below 4,000 cubic feet per second (110 m3/s). Annual runoff volume has ranged from a high of 22.2 million acre-feet (27.38 km3) in 1984 to a low of 3.8 million acre-feet (4.69 km3) in 2002, although only a small portion of this flow reaches the Gulf.
Between 85–90 percent of the Colorado River's discharge originates in snowmelt, mostly from the Rocky Mountains of Colorado and Wyoming. The three major upper tributaries of the Colorado – the Gunnison, Green and San Juan – alone deliver almost 9 million acre-feet (11 km3) per year to the main stem, mostly from snowmelt. The remaining 15 percent comes from a variety of sources, principally groundwater base flow and summer monsoon storms. The latter often produces heavy, highly localized floods on lower tributaries of the river, but does not contribute significant volumes of runoff. Most of the annual runoff in the basin occurs with the melting of Rocky Mountains snowpack beginning in April, which peaks during May and June before exhausting in late July–early August.
Flows at the mouth have steadily declined since the beginning of the 20th century, and in most years after 1960 the Colorado River has run dry before reaching the sea. Irrigation and municipal diversions, evaporation from reservoirs, and likely climate change have all contributed to this substantial reduction in flow. For example, the Gila River – formerly one of the Colorado's largest tributaries – contributes little more than a trickle in most years due to use of its water by cities and farms in central Arizona. The average flow rate at the northernmost point of the Mexico–United States border (NIB, or Northerly International Boundary), below major diversions such as the All-American Canal and Colorado River Aqueduct, is about 2,060 cubic feet per second (58 m3/s), 1.49 million acre-feet (1.84 km3) per year. In 1984, record-breaking precipitation and snowmelt greatly exceeded the storage capacity of reservoirs, and the Colorado River's average flow at the NIB reached 22,860 cubic feet per second (647 m3/s), 16.5 million acre-feet (20.35 km3), in that one year.
|Discharge of the Colorado River at selected locations|
|Grand Lake, CO||62.7||1.78||976||27.6||63.9||166||1953–2010|||
|Lee's Ferry, AZ||14,800||420||300,000||8,500||107,800||279,000||1895–2010|||
|Davis Dam, AZ–NV||14,180||402||116,000||3,300||169,300||438,000||1905–2010|||
|Parker Dam, AZ–CA||11,990||340||42,400||1,200||178,500||462,000||1935–2010|||
|Laguna Dam, AZ–CA||1,693||47.9||30,900||870||184,600||478,000||1971–2010|||
(near Andrade, CA)
|Monthly discharge of the Colorado at Lee's Ferry|
The United States Geological Survey (USGS) operates or has operated 46 stream gages to measure the discharge of the Colorado River, ranging from the headwaters near Grand Lake to the Mexico–U.S. border. The tables at right list data associated with eight of these gages. River flows as gaged at Lee's Ferry, Arizona, about halfway along the length of the Colorado and 16 miles (26 km) below Glen Canyon Dam, are used to determine water allocations in the Colorado River basin. The average discharge recorded there was approximately 14,800 cubic feet per second (420 m3/s), 10.72 million acre-feet (13.22 km3) per year, from 1921 to 2010. Prior to the closure of Glen Canyon Dam in 1964, the average discharge recorded between 1912–1962 was 17,850 cubic feet per second (505 m3/s), 12.93 million acre-feet (15.95 km3) per year.
The drainage basin or watershed of the Colorado River encompasses 246,000 square miles (640,000 km2) of southwestern North America, making it the seventh largest on the continent. About 238,600 square miles (618,000 km2), or 97.0% of the watershed, is in the United States. Parts of the U.S. states of Arizona, California, Colorado, New Mexico, Nevada, Utah, and Wyoming and the Mexican states of Baja California and Sonora are in the Colorado River watershed. Most of the basin is arid – defined by the Sonoran and Mojave Deserts and the expanse of the Colorado Plateau – although significant expanses of forest are found in the Rocky Mountains, the Kaibab, Aquarius, and Markagunt Plateaus in southern Utah and northern Arizona, the Mogollon Rim through central Arizona, and other smaller mountain ranges and sky islands. Elevations range from sea level at the Gulf of California to over 13,000 feet (4,000 m) in the mountains of Colorado and western Wyoming, with an average of 5,500 feet (1,700 m) across the entire basin.
Climate varies widely across the watershed. Mean monthly high temperatures range from 77.5 to 105 °F (25 to 41 °C) and lows from 48 to 10.5 °F (9 to -12 °C), with extremes of up to 120 °F (49 °C) in the desert regions of the watershed to −50 °F (−46 °C) in Rocky Mountain winter storms. Annual precipitation averages 6.5 in (170 mm), ranging from over 40 in (1,000 mm) in some areas of the Rockies to just 0.6 in (15 mm) along the Mexican reach of the river. The upper basin generally receives snow and rain during the winter and early spring, while the lower basin is characterized by intense but infrequent summer thunderstorms brought on by the North American Monsoon.
As of 2010, approximately 12.7 million people lived in the Colorado River basin.[n 4] Phoenix in Arizona and Las Vegas in Nevada are the largest metropolitan areas in the watershed. Population densities are also high along the lower Colorado River below Davis Dam, which includes Bullhead City, Lake Havasu City and Yuma. Other significant population centers in the basin include Tucson, Arizona, St. George, Utah and Grand Junction, Colorado. Colorado River basin states are among the fastest growing in the U.S.; the population of Nevada alone increased by about 66 percent between 1990 and 2000 as Arizona grew by some 40 percent.
The Colorado River basin shares drainage boundaries with many other major watersheds of North America. The Continental Divide of the Americas forms a large portion of the eastern boundary of the watershed, separating it from the basins of the Yellowstone River and Platte River, tributaries of the Missouri River, on the northeast, and from the headwaters of the Arkansas River on the east. Both the Missouri and Arkansas Rivers are part of the Mississippi River system. Further south, the Colorado River basin borders on the Rio Grande drainage, which along with the Mississippi flows to the Gulf of Mexico, as well as a series of endorheic (closed) drainage basins in southwestern New Mexico and extreme southeastern Arizona.
For a short stretch, the Colorado watershed meets the drainage basin of the Snake River, a tributary of the Columbia River, in the Wind River Range of western Wyoming. Southwest of there, the northern divide of the Colorado watershed skirts the edge of the Great Basin, bordering on the closed drainage basins of the Great Salt Lake and Sevier River in central Utah, and other closed basins in southern Utah and Nevada. To the west in California, the Colorado River watershed borders on those of small closed basins in the Mojave Desert, the largest of which is the Salton Sea drainage north of the Colorado River Delta. On the south, the watersheds of the Sonoyta, Concepción, and Yaqui Rivers, all of which drain to the Gulf of California, border that of the Colorado.
As recently as the Cretaceous period one hundred million years ago, much of western North America was still part of the Pacific Ocean. Tectonic forces from the collision of the Farallon Plate with the North American Plate pushed up the Rocky Mountains between 50–75 million years ago in a mountain-building episode known as the Laramide orogeny. The Colorado first formed as a west-flowing stream draining the southwestern portion of the range, and the uplift also diverted the Green River from its original course to the Mississippi River west towards the Colorado. Approximately 20–30 million years ago, volcanic activity related to the orogeny led to the Mid-Tertiary ignimbrite flare-up which created smaller formations such as the Chiricahua Mountains in Arizona, and deposited massive amounts of volcanic ash and debris over the watershed. The Colorado Plateau first began to rise during the Eocene, but did not attain its present height until about five million years ago, about when the Colorado River established its present course into the Gulf of California.
The exact nature in which the river's present course and the Grand Canyon were formed is uncertain. Before the Gulf of California was formed approximately 5–12 million years ago by faulting processes along the boundary of the North American and Pacific Plates, the Colorado flowed west to an outlet on the Pacific Ocean – possibly Monterey Bay on the Central California coast, forming the Monterey submarine canyon. The uplift of the Sierra Nevada mountains began about 4.5 million years ago, diverting the Colorado southwards towards the Gulf. As the Colorado Plateau rose between 2.5–5 million years ago, the river maintained its ancestral course (as an antecedent stream) and began to cut the Grand Canyon. Antecedence played a major part in shaping other peculiar geographic features in the watershed, including the Dolores River's bisection of Paradox Valley in Colorado and the Green River carving its way through the Uinta Mountains in Utah.
Sediments carried from the plateau by the Colorado River created a vast delta made of more than 10,000 cu mi (42,000 km3) of material that walled off the northernmost part of the gulf in approximately one million years. Cut off from the ocean, the portion of the gulf north of the delta eventually evaporated and formed the Salton Sink, which reached about 260 feet (79 m) below sea level. Between then and now the river changed course into the Salton Sink at least three times, transforming it into Lake Cahuilla, which at maximum flooded up the valley to present-day Indio, California. The lake took about 50 years to evaporate after the Colorado resumed flowing to the Gulf. The present-day Salton Sea can be considered the most recent incarnation of Lake Cahuilla, though on a much smaller scale.
Between 1.8 million and 10,000 years ago, massive flows of basalt from the Uinkaret volcanic field in northern Arizona dammed the Colorado River within the Grand Canyon. At least thirteen lava dams were formed, the largest of which was more than 2,300 feet (700 m) high, backing the river up for nearly 500 miles (800 km) to present-day Moab, Utah. The lack of associated sediment deposits along this stretch of the Colorado River, which would have accumulated in the impounded lakes over time, suggests that most of these dams did not survive for more than a few decades before collapsing or being washed away. Failure of the lava dams caused by erosion, leaks and cavitation caused catastrophic flooding which may have been some of the largest ever to occur in North America, rivaling the ice age Missoula Floods of the northwestern United States. Mapping of flood deposits indicate that crests as high as 700 feet (210 m) passed through the Grand Canyon, reaching peak discharges as great as 17 million cubic feet per second (500,000 m3/s).
Human history 
Indigenous peoples 
The first humans of the Colorado River basin were likely Paleo-Indians of the Clovis and Folsom cultures, who first arrived on the Colorado Plateau about 12,000 years ago. Very little human activity occurred in the watershed until the rise of the Desert Archaic Culture, which from 8,000 to 2,000 years ago constituted most of the region's human population. These prehistoric inhabitants led a generally nomadic lifestyle, gathering plants and hunting small animals (though some of the earliest peoples hunted larger mammals that became extinct in North America after the end of the Pleistocene epoch). Another notable early group was the Fremont culture, whose peoples inhabited the Colorado Plateau between 2,000 to 700 years ago. The Fremont were likely the first peoples of the Colorado River basin to domesticate crops and construct masonry dwellings; in addition, they have left behind a large amount of rock art and petroglyphs, many of which have survived to the present day.
Beginning in the early centuries A.D., Colorado River Basin peoples began to form large agriculture-based societies, some of which lasted hundreds of years and grew into well-organized civilizations encompassing tens of thousands of inhabitants. The Ancient Puebloan (also known as Anasazi or Hisatsinom) people of the Four Corners region were descended from the Desert Archaic culture. The Puebloans dominated the basin of the San Juan River, with the center of their civilization in northern New Mexico's Chaco Canyon. In Chaco Canyon and the surrounding lands, they built more than 150 multi-story pueblos or "great houses", the largest of which, Pueblo Bonito, is composed of more than six hundred rooms. The Hohokam culture was present along the middle Gila River beginning around 1 A.D. Between 600–700 A.D. they began to employ irrigation on a large scale, and did so more prolifically than any other native group in the Colorado River basin. An extensive system of irrigation canals was constructed on the Gila and Salt Rivers, with various estimates of a total length ranging from 180 to 300 miles (290 to 480 km) and capable of irrigating 25,000 to 250,000 acres (10,000 to 100,000 ha). Both civilizations supported large populations at their height, with the Chaco Canyon Puebloans numbering between 6,000–15,000 and estimates for the Hohokam ranging between 30,000–200,000.
These sedentary peoples heavily exploited their surroundings, practicing logging and harvesting of other resources on a large scale. The construction of irrigation canals may have led to a significant change in the morphology of many waterways in the Colorado River basin. Prior to human contact, rivers such as the Gila, Salt and Chaco were shallow perennial streams with low, vegetated banks and large floodplains. In time, flash floods caused significant downcutting on irrigation canals, which in turn led to the entrenchment of the original streams into arroyos, making agriculture difficult. A variety of methods were employed to combat these problems, including the construction of large dams, but when a megadrought hit the region in the 1300s A.D. the ancient civilizations of the Colorado River basin abruptly collapsed. Some Puebloans migrated to the Rio Grande Valley of central New Mexico and south-central Colorado, becoming the predecessors of the Hopi, Zuni, Laguna and Acoma in western New Mexico. Many of the tribes that would inhabit the Colorado River basin at the time of European contact are descended from Puebloan and Hohokam survivors, while others already had a long history of living in the region, or migrated in from bordering lands.
|Mohave: 'Aha Kwahwat|
|Havasupai: Ha Ŧay Gʼam /
The Navajo were an Athabaskan people who migrated from the north into the Colorado River basin around 1025 A.D. They soon established themselves as the dominant Native American tribe in the Colorado River basin, with their territory stretching over parts of present-day Arizona, New Mexico, Utah and Colorado in the original homelands of the Puebloans; in fact, they acquired agricultural skills from them before the collapse of their civilization in the 1300s. A profusion of other tribes have made a continued, lasting presence along the Colorado River. The Mohave have lived along the rich bottomlands of the lower Colorado below Black Canyon since 1200 A.D. They were fishermen – navigating the river on rafts made of reeds to catch Gila trout and Colorado pikeminnow – and farmers, relying on the annual floods of the river rather than irrigation to water their crops. Ute peoples have inhabited the northern Colorado River basin, mainly in present-day Colorado, Wyoming and Utah, since at least 1 A.D., but did not become well-established in the Four Corners area until 1500 A.D. The Apache, Maricopa, Pima, Havasupai and Hualapai are among many other groups that lived along or had their territories bordering on the Colorado River and its tributaries.
Beginning in the 1600s, contact with Europeans brought significant changes to the lifestyles of Native Americans in the Colorado River basin. Missionaries sought to convert indigenous peoples to Christianity, some of which met with success such as Father Eusebio Francisco Kino's 1694 encounter with the "docile Pimas of the Gila Valley [who] readily accepted Father Kino and his Christian teachings". The Spanish introduced sheep and goats to the Navajo, who came to rely heavily on them for meat, milk and wool. By the mid-1500s, the Utes, having acquired horses from the Spanish, introduced them to the Colorado River basin. The use of horses spread through the basin via trade between the various tribes and greatly facilitated hunting, communications and travel for indigenous peoples. More warlike groups such as the Utes and Navajos often used the horse to their advantage in raids against tribes that were slower to adopt horses, such as the Goshutes and Southern Paiutes.
The gradual influx of European and American explorers, fortune seekers and settlers into the region eventually led to conflicts that forced many Native Americans off their traditional lands. After the acquisition of the Colorado River basin from Mexico in the Mexican–American War in 1846, U.S. military forces commanded by Kit Carson forced more than eight thousand Navajo men, women and children from their homes after a series of unsuccessful attempts to confine their territory – many of which were met with violent resistance. In what is now known as the Long Walk of the Navajo, the captives – many of whom died along the harsh route – were marched from Arizona to Fort Sumner in New Mexico. Four years later, the Navajo signed a treaty that moved them onto a reservation in the Four Corners region that is today known as the Navajo Nation, the largest Native American reservation in the United States, encompassing 27,000 square miles (70,000 km2) with a population of over 180,000 as of 2000.
The Mohave were expelled from their territory after a series of minor skirmishes and raids on wagon trains passing through the area in the late 1850s, culminating in an 1859 battle with American forces that concluded the Mohave War. In 1870, the Mohave were relocated to a reservation at Fort Mojave, which spans the borders of Arizona, California and Nevada. Some Mohave were also moved to the 432-square-mile (1,120 km2) Colorado River Indian Reservation on the Arizona-California border, originally established for the Mohave and Chemehuevi people in 1865. In the 1940s, some Hopi and Navajo people were also relocated to this reservation. The four tribes now form a geo-political body known as the Colorado River Indian Tribes.
Water rights of Native Americans in the Colorado River basin were largely ignored during the extensive water resources development carried out on the river and its tributaries in the 19th and 20th centuries. In addition, the construction of dams has often had negative impacts on tribal peoples, such as the Chemehuevi when their riverside lands were flooded after the completion of Parker Dam in 1938. Ten Native American tribes in the basin now hold, or continue to claim, water rights to the Colorado River. The U.S. government has taken some action to help quantify and develop the water resources of Native American reservations. In fact, the first federally funded irrigation project in the U.S. was the construction of an irrigation canal on the Colorado River Indian Reservation in 1867. Other water projects include the Navajo Indian Irrigation Project, authorized in 1962 to deliver water for the irrigation of lands in part of the Navajo Nation in north-central New Mexico. The Navajo, however, continue to seek expansion of their water rights because of difficulties in water supply on their reservation – about forty percent of its inhabitants must haul water by truck many miles to their homes. In the 21st century, they have filed legal claims against the governments of Arizona, New Mexico, and Utah for increased water rights. Some of these claims have been successful for the Navajo, such as a 2004 settlement in which they received a 326,000-acre-foot (0.402 km3) allotment from New Mexico.
Early explorers 
During the 1500s, the Spanish began to explore and colonize western North America. An early motive was the search for the Seven Cities of Gold, or "Cibola", rumored to have been built by Native Americans somewhere in the desert Southwest. In 1536 Francisco de Ulloa, the first documented European to reach the Colorado River, sailed up the Gulf of California and a short distance into the river's delta. Francisco Vásquez de Coronado's 1540–1542 expedition began as a search for the fabled Cities of Gold, but after learning from natives in New Mexico of a large river to the west, he sent García López de Cárdenas to lead a small contingent to find it. With the guidance of Hopi Indians, Cárdenas and his men became the first outsiders to see the Grand Canyon. However, Cárdenas was reportedly unimpressed with the canyon, assuming the width of the Colorado River at six feet (1.8 m) and estimating 300-foot (91 m)-tall rock formations to be the size of a man. After unsuccessfully attempting to descend to the river, they left the area, defeated by the difficult terrain and torrid weather.
In 1540, Hernando de Alarcón and his fleet reached the mouth of the river, intending to provide additional supplies to Coronado's expedition. Alarcón may have sailed the Colorado as far upstream as the present-day California–Arizona border. However, Coronado never reached the Gulf of California, and Alarcón eventually gave up and left. Melchior Díaz reached the delta in the same year, intending to establish contact with Alarcón, but the latter was already gone by the time of Díaz's arrival. Díaz named the Colorado River Rio del Tizon ("Firebrand River") after seeing a practice used by the local natives for warming themselves. The name Tizon lasted for the next two hundred years, while the name Colorado ("Red River") was first applied to a tributary of the Gila River, possibly the Verde River, circa 1720. The first known map to label the main stem as the Colorado was drawn by French cartographer Jacques-Nicolas Bellin in 1743.
During the 18th and early 19th centuries, many Americans and Spanish believed in the existence of the Buenaventura River, purported to run from the Rocky Mountains in Utah or Colorado to the Pacific Ocean. The name Buenaventura was actually given to the Green River by Silvestre Vélez de Escalante as early as 1776, but Escalante did not know that the Green drained to the Colorado. Many later maps showed the headwaters of the Green and Colorado rivers connecting with the Sevier River (Rio San Ysabel) and Utah Lake (Lake Timpanogos) before flowing west through the Sierra Nevada into California. Mountain man Jedediah Smith reached the lower Colorado by way of the Virgin River canyon in 1826. Smith called the Colorado the "Seedskeedee", as the Green River in Wyoming was known to fur trappers, correctly believing it to be a continuation of the Green and not a separate river as others believed under the Buenaventura myth. John C. Frémont's 1843 Great Basin expedition proved that no river traversed the Great Basin and Sierra Nevada, officially debunking the Buenaventura myth.
In 1857, Lieutenant Joseph C. Ives led an expedition to explore the feasibility of using the Colorado River as a navigation route in the Southwest. Ives and his men used a specially built steamboat, the shallow-draft U.S.S. Explorer, and traveled up the river as far as Black Canyon on the present Arizona–Nevada border. After experiencing numerous groundings and accidents and inhibited by low water in the river, Ives declared: "Ours has been the first, and will doubtless be the last, party of whites to visit this profitless locality. It seems intended by nature that the Colorado River, along the greater portion of its lonely and majestic way, shall be forever unvisited and undisturbed."
Up until the mid-1800s, long stretches of the Colorado and Green Rivers between Wyoming and Nevada remained largely unexplored due to the remote location and dangers of navigating the river. Because of the dramatic drop in elevation of the two rivers, there were rumors of huge waterfalls and violent rapids, and Native American tales strengthened their credibility. In 1869, one-armed Civil War veteran John Wesley Powell led an expedition from Green River Station in Wyoming, aiming to run the two rivers all the way down to St. Thomas, Nevada, near present-day Hoover Dam. Powell and nine men – none of whom had prior whitewater experience – set out in May. After braving the rapids of the Gates of Lodore, Cataract Canyon and other gorges along the Colorado, the party arrived at the mouth of the Little Colorado River, where Powell noted down arguably the most famous words ever written about the Grand Canyon of the Colorado:
We are now ready to start on our way down the Great Unknown. Our boats, tied to a common stake, are chafing each other, as they are tossed by the fretful river. They ride high and buoyant, for their loads are lighter than we could desire. We have but a month's rations remaining. The flour has been resifted through the mosquito net sieve; the spoiled bacon has been dried, and the worst of it boiled; the few pounds of dried apples have been spread in the sun, and reshrunken to their normal bulk; the sugar has all melted, and gone on its way down the river; but we have a large sack of coffee. The lighting of the boats has this advantage: they will ride the waves better, and we shall have little to carry when we make a portage.
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.We have an unknown distance yet to run; an unknown river yet to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not; Ah, well! we may conjecture many things. The men talk as cheerfully as ever; jests are bandied about freely this morning; but to me the cheer is somber and the jests are ghastly.—John Wesley Powell's journal, August 1869
On August 28, 1869, three men deserted the expedition, convinced that they could not possibly survive the trip through the canyon. Conversely they were killed by either Native Americans or Mormon settlers after making it to the rim of the Grand Canyon; just two days later, the expedition ran the last of the Grand Canyon rapids and reached St. Thomas. Powell led a second expedition in 1871, this time with financial backing from the U.S. government. The explorers named many features along the Colorado and Green Rivers, including Glen Canyon, the Dirty Devil River, Flaming Gorge, and the Gates of Lodore. In what is perhaps a twist of irony, modern-day Lake Powell, which floods Glen Canyon, is also named for their leader.
American settlement 
During the Manifest Destiny era of the mid-19th century, American pioneers settled many western states but generally avoided the Colorado River basin until the 1850s. Under Brigham Young's grand vision for a "vast empire in the desert" (the State of Deseret) Mormon settlers were among the first whites to establish a permanent presence in the watershed. In 1865, Mormon colonists founded St. Thomas at the confluence of the Colorado and Virgin Rivers in Nevada. The settlement reached a peak population of about 600 before being abandoned in 1871, and thereafter the surrounding valley became a haven for outlaws and cattle rustlers.
Mormons founded the settlement of Vernal along the Green River in Utah in 1878, and populated the Little Colorado River valley later in the century, creating towns such as St. Johns, Arizona. They also established settlements along the Gila River in central Arizona beginning in 1871. These early settlers were impressed by the extensive ruins of the Hohokam civilization that previously occupied the Gila River valley, and are said to have "envisioned their new agricultural civilization rising as the mythical phoenix bird from the ashes of Hohokam society". The Mormons were the first whites to develop the water resources of the basin on a large scale, and built complex networks of dams and canals to irrigate wheat, oats and barley in addition to establishing extensive sheep and cattle ranches.
One of the main reasons the Mormons were able to colonize Arizona was the existence of Jacob Hamblin's ferry across the Colorado at Lee's Ferry (then known as Pahreah Crossing), which began running in March 1864. This location was the only section of river for hundreds of miles in both directions where the canyon walls dropped away, allowing for the development of a transport route. John Doyle Lee established a more permanent ferry system at the site in 1870. One reason Lee chose to run the ferry was to flee from Mormon leaders who held him responsible for the Mountain Meadows Massacre, in which 120 emigrants in a wagon train were killed by a local militia disguised as Native Americans. Even though it was located along a major travel route, Lee's Ferry was very isolated, and there Lee and his family established the aptly named Lonely Dell Ranch. In 1928, the ferry sank resulting in the deaths of three men. Later that year, the Navajo Bridge was completed at a point five miles (8 km) downstream, rendering the ferry obsolete.
Gold strikes from the mid-1800s through the early 1900s played a major role in attracting settlers to the upper Colorado River basin. In 1859, a group of adventurers from Georgia discovered gold along the Blue River in Colorado and established the mining boomtown of Breckenridge. During 1875, even bigger strikes were made along the Uncompahgre and San Miguel Rivers, also in Colorado, and these led to the creation of Ouray and Telluride, respectively. Because most gold deposits along the upper Colorado River and its tributaries occur in lode deposits, extensive mining systems and heavy machinery were required to extract them. Mining remains a substantial contributor to the economy of the upper basin and has led to acid mine drainage problems in some regional streams and rivers.
Starting in the latter half of the 19th century, the lower Colorado below Black Canyon became an important waterway for steamboat commerce. In 1852, the Uncle Sam was launched to provide supplies to the U.S. Army outpost at Fort Yuma. Although this vessel struck a sandbar and sank early in its career, commercial traffic quickly proliferated because river transport was much cheaper than hauling freight over land. Navigation on the Colorado River was dangerous because of the shallow channel and flow variations, and the first sternwheeler on the river, the Colorado of 1855, was designed to carry 60 short tons (54 t) while drawing less than 2 feet (0.61 m) of water. Steamboats quickly became the principal source of communication and trade along the river until competition from railroads began in the 1870s, and finally the construction of dams along the lower river, none of which had locks to allow the passage of ships.
Grand River controversy 
Prior to 1921, the upper Colorado above the confluence with the Green River in Utah was not considered part of the main stem. This section of the Colorado had been known as the Grand River since 1836. In 1921, U.S. Representative Edward T. Taylor of Colorado petitioned the Congressional Committee on Interstate and Foreign Commerce to rename the Grand River as the Colorado River. Taylor saw the fact that the Colorado River started outside the border of his state as an "abomination". On July 25 the name change was made official in House Joint Resolution 460 of the 66th Congress, over the objections of representatives from Wyoming, Utah and the USGS, which noted that the Green River was much longer and had a larger drainage basin above its confluence with the Grand, although the Grand River frequently contributed a greater flow of water.
Engineering and development 
With 36 to 40 million people dependent on its water for both agricultural and domestic needs, the Colorado River is considered one of the "most controlled, controversial and litigated rivers in the world". Over 29 major dams and hundreds of miles of canals serve to supply thirsty cities, provide irrigation water to some four million acres (16,000 km2), and generate more than 12 billion kWh of hydroelectricity each year. Often called "America's Nile", the Colorado is so carefully managed – with basin reservoirs capable of holding four times the river's annual flow – that each drop of its water is used an average of seventeen times in a single year.
One of the earliest water projects in the Colorado River basin was the Grand Ditch, a 16-mile (26 km) diversion canal that sends water from the Never Summer Mountains, which would naturally have drained into the headwaters of the Colorado River, to bolster supplies in Colorado's Front Range Urban Corridor. Constructed primarily by Japanese and Mexican laborers, the ditch was considered an engineering marvel when completed in 1890, delivering 17,700 acre feet (0.0218 km3) across the Continental Divide each year. Because roughly 75 percent of Colorado's precipitation falls west of the Rocky Mountains while 80 percent of the population lives east of them, more of these interbasin water transfers, locally known as transmountain diversions, followed. While first envisioned in the late 19th century, construction on the Colorado-Big Thompson Project (C-BT) did not begin until the 1930s. The C-BT now delivers more than eleven times the flow through the Grand Ditch from the Colorado River watershed to cities along the Front Range.
Meanwhile, large-scale development was also beginning on the opposite end of the Colorado River. In 1900, entrepreneurs of the California Development Company (CDC) looked to the Imperial Valley of southern California as an excellent location to develop agriculture irrigated by the waters of the river. Engineer George Chaffey was hired to design the Alamo Canal, which split off from the Colorado River near Pilot Knob, curved south into Mexico, and dumped into the Alamo River, a dry arroyo which had historically been observed to carry flood flows of the Colorado into the Salton Sink. With a stable year-round flow in the Alamo River, irrigators in the Imperial Valley were able to begin large-scale farming and small towns in the region started to expand with the influx of job-seeking migrants. By 1903, more than 100,000 acres (40,000 ha) in the valley were under cultivation, supporting a growing population of four thousand.
It was not long before the Colorado River began to wreak havoc with its erratic flows. In autumn, the river would drop below the level of the canal inlet and temporary brush diversion dams had to be constructed. In early 1905, heavy floods destroyed the headworks of the canal and water began to flow uncontrolled down the canal towards the Salton Sink. On August 9, the entire flow of the Colorado swerved into the canal and began to flood the bottom of the Imperial Valley. In a desperate gamble to close the breach, crews of the Southern Pacific Railroad, whose tracks ran through the valley, attempted to dam the Colorado above the canal only to see their work demolished by a flash flood. It took seven attempts, over US$3 million and two years for the railroad, the CDC and the federal government to permanently block the breach and send the Colorado on its natural course back to the gulf – but not before part of the Imperial Valley was flooded under a 45-mile-long (72 km) lake, today's Salton Sea. After the immediate flooding threat passed, it was realized that a more permanent solution would be needed to rein in the Colorado.
In 1922, six U.S. states in the Colorado River basin signed the Colorado River Compact, which divided half of the river's flow to both the Upper Basin (the drainage area above Lee's Ferry, comprising parts of Colorado, New Mexico, Utah and Wyoming and a small portion of Arizona) and the Lower Basin (Arizona, California, Nevada, and parts of New Mexico and Utah). Each was given rights to 7,500,000 acre feet (9.3 km3) of water per year, a figure believed to represent half of the river's minimum flow at Lee's Ferry. This was followed by a U.S.–Mexico treaty in 1944, allocating 1,500,000 acre feet (1.9 km3) of Colorado River water to the latter country per annum. In addition, Arizona did not ratify the Colorado River Compact until 1944 because it feared that California would take too much of the lower basin allotment and leave little left over for Arizona's use. These and nine other decisions, compacts, federal acts and agreements made between 1922 and 1973 form what is now known as the Law of the River.
On September 30, 1935, the United States Bureau of Reclamation (USBR) completed Hoover Dam in the Black Canyon of the Colorado River. Behind the dam rose Lake Mead, the largest artificial lake in the U.S, capable of holding more than two years of the Colorado's flow. The construction of Hoover was a major step towards stabilizing the lower channel of the Colorado River, storing water for irrigation in times of drought, and providing much-needed flood control. Hoover was the tallest dam in the world at the time of construction, and also had the world's largest hydroelectric power plant. Flow regulation from Hoover Dam opened the doors for rapid development on the lower Colorado River: Imperial and Parker Dams followed in 1938, and Davis Dam was completed in 1950.
Completed in 1938 some 20 miles (32 km) above Yuma, Imperial Dam diverts nearly all of the Colorado's flow into two irrigation canals. The All-American Canal, built as a permanent replacement for the Alamo Canal, is so named because it lies completely within the U.S., unlike its ill–fated predecessor. With a capacity of over 26,000 cu ft/s (740 m3/s), the All-American is the largest irrigation canal in the world, supplying water to 500,000 acres (2,000 km2) of California's Imperial Valley. Because the valley's warm and sunny climate lends to a year-round growing season in addition to the large water supply furnished by the Colorado, the Imperial Valley is now one of the most productive agricultural regions in North America. In 1957, the USBR completed a second canal, the Gila Gravity Main Canal, to irrigate about 110,000 acres (450 km2) in southwestern Arizona with Colorado River water as part of the Gila Project.
|Colorado River water allocations|
The Lower Basin states also sought to develop the Colorado for municipal supplies. Providing water for up to 10 million people each year, the Colorado River Aqueduct, which delivers water nearly 250 miles (400 km) from near Parker Dam to California's Los Angeles metropolitan area, was completed in 1941. The San Diego Aqueduct branch, whose initial phase was complete by 1947, furnishes water to nearly three million people in San Diego and its suburbs. The Las Vegas Valley of Nevada experienced rapid growth in part due to Hoover Dam construction, and tapped a pipeline into Lake Mead by 1937. Nevadan officials, believing that groundwater resources in the southern part of the state were sufficient for future growth, were more concerned with securing a large amount of the dam's power supply than water from the Colorado; thus they settled for the smallest allocation of all the states in the Colorado River Compact.
Central Arizona initially relied on the Gila River and its tributaries through projects such as the Theodore Roosevelt and Coolidge Dams – completed in 1924 and 1928, respectively. Roosevelt was the first large dam constructed by the USBR, and provided the initial water supplies needed to start off large-scale agricultural and urban development in the region. Agricultural and urban growth in Arizona eventually outstripped the capacity of local rivers, leading to the inception of the Central Arizona Project in 1968, which now irrigates more than 830,000 acres (3,400 km2) and provides municipal supplies to over five million people from Phoenix to Tucson using water from the Colorado River.
Through the early decades of the 20th century, the Upper Basin states with the exception of Colorado remained relatively undeveloped and utilized little of the water allowed to them under the Colorado River Compact. However, water use had increased significantly by the 1950s, and more water was being diverted out of the Colorado River basin to the Front Range corridor, the Salt Lake City area in Utah, and the Rio Grande basin in New Mexico. Such projects included the Roberts Tunnel, completed in 1956, which diverts 63,000 acre feet (0.078 km3) per year from the Blue River to the city of Denver, and the Fryingpan-Arkansas Project, which delivers 69,200 acre feet (0.0854 km3) from the Fryingpan River to the Arkansas River basin each year. Without the addition of surface water storage in the upper basin, there was no guarantee that the upper basin states would be able to utilize the full amount of water given to them by the compact. There was also the concern that drought could impair the upper basin's ability to deliver the required 7,500,000 acre feet (9.3 km3) past Lee's Ferry per year as stipulated by the compact. A 1956 act of Congress cleared the way for the USBR's Colorado River Storage Project (CRSP), which entailed the construction of large dams on the Colorado, Green, Gunnison and San Juan Rivers.
The initial blueprints for the CRSP included two dams on the Green River within Dinosaur National Monument's Echo Park Canyon, a move criticized by both the U.S. National Park Service and environmentalist groups such as the Sierra Club. Controversy reached a nationwide scale, and the USBR dropped its plans for the Dinosaur dams in exchange for a dam at Flaming Gorge and a raise to an already-proposed dam at Glen Canyon. The famed opposition to Glen Canyon Dam, the primary feature of the CRSP, did not build momentum until construction was well underway. This was primarily because of Glen Canyon's remote location and the result that most of the American public did not even know of the existence of the impressive gorge; the few who had, however, contended that it had much greater scenic value than Echo Park. Sierra Club leader David Brower fought the dam both during the construction and for many years afterwards until his death in 2000. Brower strongly believed that he was personally responsible for the failure to prevent Glen Canyon's flooding, calling it his "greatest mistake, greatest sin".
Environmental impacts 
Historically, the Colorado transported between 85 and 100 million short tons (77–91 million t) of sediment or silt to the Gulf of California each year – second only to the Mississippi among North American rivers. This sediment nourished wetlands and riparian areas along the river's lower course, particularly in its 3,000-square-mile (7,800 km2) delta, once the largest desert estuary on the continent. Currently, the majority of sediments carried by the Colorado River are deposited at the upper end of Lake Powell, and most of the remainder ends up in Lake Mead. Various estimates place the time it would take for Powell to completely fill with silt at three to seven hundred years. Dams trapping sediment not only pose damage to river habitat but also threaten future operations of the Colorado River reservoir system.
Reduction in flow caused by dams, diversions and evaporation losses from reservoirs – the latter of which reduces the river's runoff by more than fifteen percent – has had severe ecological consequences in the Colorado River Delta and the Gulf of California. Historically, the delta with its large freshwater outflow and extensive salt marshes provided an important breeding ground for aquatic species in the Gulf. 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. Salinity in the lower Colorado River has also increased as a result of reduced flows. The lower Colorado's salt content was about 50 parts per million (ppm) in its natural state, but by the 1960s, it had increased to well over 2000 ppm. In 1997, the USBR estimated that saline irrigation water caused crop damages exceeding $500 million in the U.S. and $100 million in Mexico. Efforts have been made to combat the salt issue in the lower Colorado, including the construction of a desalination plant at Yuma.
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. The Colorado's average temperature once ranged from 85 °F (29 °C) at the height of summer to near freezing, 32 °F (0 °C), in winter – but modern flows through the Grand Canyon, for example, rarely deviate significantly from 46 °F (8 °C). Changes in temperature regime have caused declines of native fish populations, and stable flows enable increased vegetation growth, obstructing riverside habitat. 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.
Uncertain future 
|“||[The Colorado is] a 'deficit' river, as if the river were somehow at fault for its overuse.||”|
When the Colorado River Compact was drafted in the 1920s, it was based on barely thirty years of streamflow records that suggested an average annual flow of 17.5 million acre-feet (21.59 km3) past Lee's Ferry. Modern studies of tree rings revealed that those three decades were probably the wettest in the past 500–1,200 years – and that the long-term annual flow past Lee's Ferry is probably closer to 13.5 million acre-feet (16.65 km3).[n 6] This has resulted in more water being allocated to river users than actually flows through the Colorado. Droughts have exacerbated the issue of water over-allocation, including one in the 1950s, which saw several consecutive years of notably low water and has often been used in planning for "a worst-case scenario".
The most severe drought on record began in the early 21st century, in which the river basin only produced normal or above-average runoff in three years between 2000 and 2012. Major reservoirs in the basin dropped to historic lows, with Lake Powell falling to just one-third of capacity in early 2005, the lowest level on record since the reservoir was first filling in 1969. The watershed is currently experiencing a warming trend, which is accompanied by earlier snowmelt and a general reduction in precipitation. A 2004 study showed that a 1–6% decrease of precipitation would lead to runoff declining by as much as eighteen percent by 2050. Average reservoir storage would decline by at least 32%, further crippling the region's water supply and hydropower generation. A study by the Scripps Research Institute in 2008 predicted that both Lake Mead and Lake Powell stand a fifty-fifty chance of dropping to useless levels or "dead pool"[n 7] by 2021 if current drying trends and water usage rates continue.
In late 2010, Lake Mead dropped to just 8 feet (2.4 m) above the first "drought trigger" elevation, a level at which Arizona and Nevada would have to begin rationing water as delineated by the Colorado River Compact. Despite above-average runoff in 2011 that raised the immense reservoir more than 30 feet (9.1 m), officials estimate that the influx will only stave off rationing until about 2016. Rapid development and economic growth further complicate the issue of a secure water supply, particularly in the case of California and Nevada fighting over the yet unused portion of Arizona's share of the Colorado. Although stringent water conservation measures have been implemented, the threat of severe shortfalls in the Colorado River basin continues to increase each year.
Wildlife and plants 
The Colorado River and its tributaries often nourish extensive corridors of riparian growth as they traverse the arid desert regions of the watershed. Although riparian zones represent a relatively small proportion of the basin and they have been affected by engineering projects and river diversion in many places, they have the greatest biodiversity of any habitat in the basin. The most prominent riparian zones along the river occur along the lower Colorado below Davis Dam, especially in the Colorado River Delta, whose riparian areas support 358 species of birds despite the reduction in freshwater flow and invasive plants such as tamarisk (salt cedar). Reduction of the delta's size has also threatened animals such as jaguars and the vaquita porpoise, which is endemic to the gulf. However, human development of the Colorado River has also helped to create new riparian zones by smoothing out the river's seasonal flow rhythms, notably through the Grand Canyon.
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. Before logging in the 19th century, forests were abundant in high elevations as far south as the Mexico–U.S. border, and runoff from these areas nourished abundant grassland communities in river valleys. Some arid regions of the watershed, such as the upper Green River valley in Wyoming, Canyonlands National Park in Utah and the San Pedro River valley in Arizona and Sonora, supported extensive reaches of grassland roamed by large mammals such as buffalo and antelope as late as the 1860s. Near Tucson, Arizona, "where now there is only powder-dry desert, the grass once reached as high as the head of a man on horse back".
Rivers and streams in the Colorado basin were once home to 49 species of native fish, of which 42 are endemic. Engineering projects and river regulation have led to the extinction of four species and severe declines in the populations of 40 species. Bonytail chub, razorback sucker, Colorado pikeminnow, and humpback chub are among those considered the most at risk; all are unique to the Colorado River system and well adapted to the river's natural silty conditions and flow variations. Clear, cold water released by dams has significantly changed characteristics of habitat for these and other Colorado River basin fishes. Additionally, about 40 fish species – notably the brown trout – were introduced during the 19th and 20th centuries, mainly for sport fishing.
Famed for its dramatic rapids and canyons, the Colorado is one of the most desirable whitewater rivers in the United States and its Grand Canyon section – run by more than 22,000 people annually – has been called the "granddaddy of rafting trips". Grand Canyon trips typically begin at Lee's Ferry and take out at Diamond Creek or Lake Mead, and range from one to eighteen days for commercial trips and from two to twenty-five days for private trips. Private (noncommercial) trips are extremely difficult to arrange because the National Park Service limits river traffic for environmental purposes; people who desire such a trip often have to wait more than ten years for the opportunity.
Several other sections of the river and its tributaries are popular whitewater runs, and many of these are also served by commercial outfitters. The Colorado's Cataract Canyon and many reaches in the Colorado headwaters are even more heavily used than the Grand Canyon, with more than 60,000 boaters running a single 4.5-mile (7.2 km) section above Radium, Colorado each year. The upper Colorado also includes many of the river's most challenging rapids, including those in Gore Canyon, which is considered so dangerous that "boating is not recommended". Another section of the river above Moab, known as the Colorado "Daily" or "Fisher Towers Section", is the most visited whitewater run in Utah, with more than 77,000 visitors in 2011 alone. The rapids of the Green River's Gray and Desolation Canyons and the less difficult "Goosenecks" section of the lower San Juan River are also frequently traversed by boaters.
Eleven U.S. national parks – Arches, Black Canyon of the Gunnison, Bryce Canyon, Canyonlands, Capitol Reef, Grand Canyon, Mesa Verde, Petrified Forest, Rocky Mountain, Saguaro, and Zion – are in the watershed, in addition to many national forests, state parks and recreation areas. Hiking, backpacking, camping, skiing and fishing are among the multiple recreation opportunities offered by these areas. However, fisheries have declined in many streams in the watershed, especially in the Rocky Mountains, because of polluted runoff from mining and agricultural activities. The Colorado's major reservoirs are also heavily traveled summer destinations. Houseboating and water-skiing are popular activities on Lakes Mead, Powell, Havasu and Mojave, as well as Flaming Gorge Reservoir in Utah and Wyoming, and Navajo Reservoir in New Mexico and Colorado. Lake Powell and surrounding Glen Canyon National Recreation Area receives more than two million visitors per year as of 2007, while nearly 7.9 million people visited Lake Mead and the Lake Mead National Recreation Area in 2008.
Major tributaries 
The Colorado is joined by over twenty-five significant tributaries, of which the Green River is the largest by both length and discharge, taking drainage from the Wind River Range of west-central Wyoming, Utah's Uinta Mountains and the Rockies of northwestern Colorado. The Gila River is the second longest and drains a greater area, but even in its natural state it averaged less than a third of the Green's flow because of its significantly more arid watershed. Both the Gunnison and the San Juan Rivers, which derive most of their water from Rocky Mountains snowmelt, contribute more water than the Gila did naturally.
|Statistics of the Colorado's longest tributaries|
|Green River||UT||730||1,170||48,100||125,000||6,048||171.3[n 8]|
|Gila River||AZ||649||1,044||58,200||151,000||247||7.0[n 9]|
|San Juan River||UT||383||616||24,600||64,000||2,192||62.1[n 10]|
|Little Colorado River||AZ||356||573||26,500||69,000||424||12.0|
|Virgin River||NV||162||261||13,020||33,700||239||6.8[n 11]|
See also 
- Colorado Desert
- List of Colorado River rapids and features
- List of largest reservoirs in the United States
- List of longest main-stem rivers in the United States
- List of longest rivers of Mexico
- Moab uranium mill tailings pile
- New London Bridge
- Upper Colorado River Endangered Fish Recovery Program
- The combined natural flows of the Colorado River at Lee's Ferry, Arizona and the Gila River at the mouth are about 15.1 million acre-feet (18.63 km3 per year. Although this amounts to most of the river's natural discharge, several significant tributaries do enter between Lee's Ferry and the Gila River, including the Little Colorado, Virgin and Bill Williams Rivers. These tributaries combined contribute an estimated 561,000 acre feet (0.69 km3) each year, bringing the total annual discharge from the entire river system to 15.7 million acre-feet (19.32 km3), corresponding to an average flow of 21,700 cu ft/s (610 m3/s).
- See note 1.
- NIB = "Northerly International Boundary", or the point at which the Colorado begins to form the Mexico–U.S. border, south of Yuma. Also note that the SIB ("Southerly International Boundary") is the point at which the Colorado ceases to form the border and passes entirely into Mexico.
- American population (9.7 million) calculated from statistics from the U.S. Census Bureau and the State of Colorado. The population in Mexico is about 3 million.
- 1 MAF=1,000,000 acre-feet (1.23 km3)
- The discrepancy between the natural flow at Lee's Ferry (13.5 million acre-feet/16.65 km3) and the gaged flow between 1921 and 2010 (10.7 million acre-feet/13.22 km3) is due mainly to water diversions above Lee's Ferry and evaporation from reservoirs, especially Lake Powell.
- Dead pool refers to the lowest lake level at which water can be released through the dam. For example, Lake Mead's "dead" capacity is about 2 million acre-feet (2.5 km3).
- Discharge data is for Green River, Utah, 117.6 miles (189.3 km) upstream from the mouth. The stream gage here measures flow from an area of 44,850 sq mi (116,200 km2), representing about 93.2% of the basin.
- Before large irrigation and municipal diversions, the Gila River discharged about 1.3 million acre-feet (1.6 km3) per year, equating a flow of nearly 2,000 cu ft/s (57 m3/s).
- Discharge data is for Bluff, Utah, located about 113.5 mi (182.7 km) above the confluence with the Colorado. The gage measures flow from an area of 23,000 sq mi (60,000 km2), about 93.5% of the basin.
- Discharge data is for Littlefield, Arizona, about 66 mi (106 km) from the confluence with the Colorado, and also upstream of the confluence with its major tributary, the Muddy River. The gage measures flow from an area of 5,090 sq mi (13,200 km2), about 39.1% of the total basin.
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- Lingenfelter, Richard E. (1978). Steamboats on the Colorado River, 1852–1916. University of Arizona Press. ISBN 0-8165-0650-7.
- Logan, Michael F. (2006). Desert Cities: The Environmental History of Phoenix and Tucson. University of Pittsburgh Press. ISBN 0-8229-4294-1.
- Nobles, Gregory H. (1998). American Frontiers: Cultural Encounters and Continental Conquest. Macmillan. ISBN 0-8090-1602-8.
- Patten, Michael A.; McCaskie, Guy; Unitt, Philip (2003). Birds of the Salton Sea: status, biogeography and ecology. University of California Press. ISBN 0-520-23593-2.
- Powell, Allan Kent (2003). The Utah Guide (3 ed.). Fulcrum Publishing. ISBN 1-55591-114-5.
- Prisciantelli, Tom (2002). Spirit of the American Southwest: Geology, Ancient Eras and Prehistoric People, Hiking Through Time. Sunstone Press. ISBN 0-86534-354-3.
- Pritzker, Barry (1998). Native Americans: An Encyclopedia of History, Culture, and Peoples 1. ABC-CLIO. ISBN 0-87436-836-7.
- Reisner, Marc (1993). Cadillac Desert. Penguin. ISBN 0-14-017824-4.
- Rolle, Andrew (1999). John Charles Fremont: Character As Destiny. University of Oklahoma Press. ISBN 0-8061-3135-7.
- Schmidt, Jeremy (1993). Grand Canyon National Park: A Natural History Guide. Houghton Mifflin Harcourt. ISBN 0-395-59932-6.
- Wildfang, Frederic B. (2005). Lake Havasu City. Images of America. Arcadia Publishing. ISBN 0-7385-3012-3.
- Wiltshire, Richard L.; Gilbert, David R.; Rogers, Jerry R., ed. (2010). Hoover Dam 75th Anniversary History Symposium: Proceedings of the Hoover Dam 75th Anniversary History Symposium, October 21–22, 2010, Las Vegas, Nevada. ASCE Publications. ISBN 0-7844-1141-7.
- Young, Richard A.; Spamer, Earle E. (2001). The Colorado River: Origin and Evolution. Grand Canyon Association. ISBN 0-938216-79-1.
Further reading 
- Darrah, William Culp, Ralph V. Chamberlin, and Charles Kelly, editors. (2009). The Exploration of the Colorado River in 1869 and 1871–1872: Biographical Sketches and Original Documents of the First Powell Expedition of 1869 and the Second Powell Expedition of 1871–1872. ISBN 978-0-87480-963-3.
- DeBuys, William (2011). A Great Aridness: Climate Change and the Future of the American Southwest. Oxford University Press. ISBN 978-0-19977-892-8.
- Fleck, Richard F., editor. (2000). A Colorado River Reader. ISBN 978-0-87480-647-2.
- Fowler, Don D., editor. (2012). Cleaving an Unknown World: The Powell Expeditions and the Scientific Exploration of the Colorado Plateau. ISBN 978-1-60781-146-6.
- Gregory, Herbert E., William Culp Darrah, and Charles Kelly, editors. (2009). The Exploration of the Colorado River and the High Plateaus of Utah by the Second Powell Expedition of 1871–1872. ISBN 978-0-87480-964-0.
- Martin, Russell (1990). A Story That Stands Like A Dam: Glen Canyon and the Struggle for the Soul of the West (1 ed.). Henry Holt and Company. ISBN 0-8050-0822-5.
|Wikimedia Commons has media related to: Colorado River|
|Wikisource has the text of the 1921 Collier's Encyclopedia article Colorado River.|
- Agriculture in the Colorado River Basin
- Colorado River Foundation
- Colorado River water allocations by state
- Drought in the Upper Colorado River Basin
- Living Rivers: Colorado Riverkeeper
- Water level data for major Colorado River reservoirs
- Where The Colorado Runs Dry