Colorado River

For other rivers named Colorado, see Colorado River (disambiguation).

The Colorado River is the principal river of the southwestern United States and northwest Mexico. The 1,450-mile (2,330 km) river flows from the Rocky Mountains to the Gulf of California, draining 246,000 square miles (640,000 km²) in parts of seven U.S. and two Mexican states. Known for its dramatic canyons and whitewater rapids, the Colorado is a vital source of water for agricultural and urban areas as well as an important provider of hydroelectric power in the southwestern desert lands of North America.

The river basin has been inhabited by humans for at least eight thousand years. Due to the area's dry climate, they practiced farming and irrigation more prolifically than other native peoples of the continent. Before the first Europeans arrived in the 1500s, many of these indigenous societies had collapsed due to either drought or poor agricultural practices. Through the next few centuries, the watershed became part of New Spain and early Mexico before the American acquisition of the region in 1848. The Colorado remained one of the last uncharted major rivers in the U.S. until the famed 1869 Powell Expedition, whose members were the first to run the river through the Grand Canyon. American settlers did not establish a large permanent presence in the watershed until the 19th and early 20th centuries.

As the Southwest's only significant source of water, the Colorado was heavily developed in the twentieth century through a system of dams, reservoirs and canals. These works irrigate some of the most productive agricultural regions in North America and supply almost 40 million people both inside and outside the watershed, whose shares are carefully managed according to a series of treaties collectively known as the "Law of the River". However, declines in runoff and heavy water use have caused over-allocation of the Colorado, a river already considered among the most regulated in the world. Overdraft of the Colorado River could lead to severe shortages by the mid–21st century, greatly endangering power generation and water supply.

Course

Main article: Course of the Colorado River

Colorado River in the Grand Canyon seen from Hermit's Rest

The Colorado rises at La Poudre Pass east of the Never Summer Mountains in the Colorado Rockies, some 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 nearly 200 miles (320 km) downstream in Arizona.

In Arizona, the Colorado passes Lee's Ferry, 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, Nevada and Utah – the river turns sharply south. As it enters the Lower Colorado River Valley, the Colorado delineates the Arizona–Nevada and Arizona–California borders 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, about 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. Above Lake Mead, most of the Colorado 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 152 m) wide, compared with 500 to 1,000 feet (150 to 300 m) for the lower river. The river is relatively shallow, with an average depth of 6 to 20 feet (1.8 to 6.1 m). Prior to the mid–20th century, the estuary of the Colorado River was 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.

Discharge

Discharge of the Colorado River at selected locations
Gage	Discharge (average)		Discharge (max)		Drainage area		Period of record	Ref
	cfs	m3/s	cfs	m3/s	mi2	km2
Grand Lake, CO	62.7	1.78	976	27.6	63.9	166	1953–2010
Dotsero, CO	2,095	59.3	22,200	630	4,390	11,400	1941–2011
Cisco, UT	7,181	203.3	125,000	3,500	24,100	62,000	1895–2010
Lee's Ferry, AZ	17,850	505	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
NIB (near Andrade, CA)	2,059	58.3	40,600	1,150	238,700	618,000	1950–2010

Monthly discharge of the Colorado at Lee's Ferry
Month		Jan.	Feb.	Mar.	Apr.	May	Jun.	Jul.	Aug.	Sep.	Oct.	Nov.	Dec.
Discharge	cfs	9,650	9,740	10,500	16,000	28,000	32,800	18,300	13,200	10,900	9,530	9,620	9,440
	m3/s	273.3	275.8	297.3	453.1	792.9	928.8	518.2	373.8	308.7	269.9	272.4	267.3

The United States Geological Survey (USGS) operates or has operated 46 stream gages along 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.

Watershed

The drainage basin or watershed of the Colorado River encompasses 246,000 sq mi (640,000 km²) of southwestern North America, making it the seventh largest on the continent. About 238,600 sq mi (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 some 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 (25.3 to 40.6 °C) and lows from 48 to 10.5 °F (8.9 to −11.9 °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.

The river in western Colorado, with the California Zephyr running alongside

As of 2010, approximately 12.7 million people lived in the Colorado River basin. 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.

Snowmelt provides between 65 to 70 percent of the Colorado River's flow, almost 90 percent of which comes from the Upper Basin states of Colorado and Wyoming. In its natural state, most of the river's flow occurred between April and July. Historically the Colorado River discharged about 15.1 million acre-feet (18.6 km³) each year, corresponding to an average flow of 20,900 cu ft/s (590 m³/s). Since the beginning of the 20th century, flows have steadily declined, and in most years after 1960 the Colorado River has run dry before reaching the sea. This substantial reduction in flow can be attributed to irrigation and municipal diversions, evaporation from reservoirs, and likely climate change. 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 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.

Geology

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.

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 two million years.

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. Most of these dams were unstable, and may have survived for as little as several hundred years before catastrophically collapsing. The main evidence pointing towards the dams' short lifespans is the lack of associated sediment deposits along this stretch of the Colorado River, which would have accumulated in the dammed lakes over time. 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).

History

Indigenous peoples

Pueblos and cliff dwellings such as this one in New Mexico were inhabited by people of the Colorado River basin in the early centuries A.D.

Nomadic people descended from Uto-Aztecan speakers have sparsely populated the Colorado Plateau for at least 8,000 years, and perhaps for as long as 12,000 years. In about 6000 B.C. the Hisatsinom, predecessor of the Puebloan peoples, settled near the Four Corners region around the San Juan River, while around 500 B.C., people of the Fremont culture began to populate the Green River valley and the eastern part of the Great Basin. It was not until about 700 A.D. that the Puebloans began to cultivate crops for food, and they did not build the large cliff dwellings for which they are famous until after 1000 A.D.

Tribes of the Colorado River basin had many different names for the river. The Maricopa tribe knew the river as Xakxwet, while the Mohave called the river 'Aha Kwahwat. The Navajo called the river Tó Ntsʼósíkooh, and both Ha Ŧay Gʼam and Sil Gsvgov were used by the Havasupai to refer to the river.

Prior to European contact, native peoples such as the Anasazi, Hohokam, Mohave, Navajo, and Pueblo lived on the arid lands of the Colorado River basin, using water from the Colorado and its tributaries for farming. Beginning circa 200 B.C., native groups formed large agriculture-based societies which lasted for hundreds of years. The Anasazi inhabited the northern part of the Colorado Plateau and the foothills of the Rockies, in areas such as Chaco Canyon near the San Juan River, while the Hohokam lived in present-day central Arizona in the Gila River valley around the confluence with the Salt River. Mohave territory comprised a wide swath of land along the lower Colorado between Black Canyon and the present–day Palo Verde Valley. Both the Anasazi and Hohokam utilized extensive systems of canals to water their farmland, with over 210 miles (340 km) built by the latter alone.

Native civilization reached its peak from 600 to 900 A.D., but began a general decline afterwards. Around the 13th century and 14th century, the Anasazi and Hohokam cultures disappeared altogether from the lands around the Colorado River. Their demise is believed to be linked to the overuse of water for irrigation combined with a severe half-century drought. Intensive irrigation lowered the water table while increasing the salinity of the soils used for farming, eventually making them unsuitable. Flash floods during the 12th century and 13th century were probably responsible for the demise of the Chaco Canyon settlements, and the Navajo and Puebloan people who lived in the high desert in the Four Corners region near present-day Lake Powell declined in numbers as well.

It is believed that many of the peoples that once inhabited the upper regions of the Colorado watershed migrated to the Rio Grande valley and the Painted Desert in the Little Colorado River region from the 13th century through the 15th century. Many of the people in the Hopi and Zuni tribes of the Sonoran Desert, as well as the Acoma and Laguna people on the Rio Grande, are believed to be descended from the ancient inhabitants of the northeastern Colorado Plateau. Other tribes, including the Mohave, who depended on the annual flooding of the Colorado River rather than irrigation to water their farms, experienced no significant population declines until European conquest.

Exploration

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.

Frederic Remington, "Coronado Sets Out To The North" c. 1905

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.

Marble Canyon, one of the many gorges that Powell's expedition traversed

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 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. Ironically, 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 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.

Settlers and miners

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.

John D. Lee, date and photographer unknown. He established a permenant ferry across the Colorado.

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 among the first 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 rebel 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 through the 1800s and 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.

The steamboat Mohave No. 2 at Yuma, c. 1876

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 ft (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.

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 and Utah and the United States Geological Survey, which noted that the Green River was much longer and had a larger drainage basin above its confluence with the Grand, although the Grand carried a slightly higher volume of water.

Engineering and development

Lake Powell, with a capacity of more than 24 million acre feet (30 km³), is the second largest reservoir on the Colorado

The Colorado River is the sole source of water for the Imperial Valley, one of California's largest agricultural regions

See also: International Boundary and Water Commission and William Mulholland

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.

In 1890, the first water flowed through 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 at the time, 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 it, 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 in 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.

"[The undammed Colorado River is] useless to anyone... I've seen all the wild rivers I ever want to see."

—Floyd Dominy, USBR commissioner from 1959–1969

It was not long before the Colorado River began to wreak havoc with its erratic flows. In the autumns of most years, the river dropped 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 mi (72 km) long 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, the seven U.S. states of 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. 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.

Hoover Dam releasing water

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, which was then and still is the largest artificial lake in the U.S. 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 boasted 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 an 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
User	Amount (MAF)	Percent
United States	15.0	90.9%
CA	4.4	26.7%
CO	3.88	23.5%
AZ	2.8	17.0%
UT	1.72	10.4%
WY	1.05	6.4%
NM	0.84	5.1%
NV	0.3	1.8%
Mexico	1.5	9.1%
Total	16.5	100%

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, by the 1950s urban and agricultural development was beginning to proliferate, 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. 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. 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 thus most of the American public did not even know of the existence of the impressive gorge; the few who had seen it knew 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

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 such as here in lower Glen Canyon.

Historically, the Colorado transported between 85–100 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 from three to seven hundred years. Dams trapping sediment pose not only damage to river habitat but also threatens 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 $500 million of crop damage in the U.S. and more than $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

Lake Mead in 2010, showing the "bathtub ring" left behind by low water levels

When the Colorado River Compact was first signed in 1922, it was based on barely thirty years of streamflow records that suggested an average annual flow of 17.5 million acre-feet (21.6 km³) past Lee's Ferry. Modern tree ring studies revealed that these 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 (16.7 km³). This has resulted in more water being allocated to river users than actually flows through the Colorado. As Marc Reisner, author of Cadillac Desert puts it, "[the Colorado is] a 'deficit' river, as if the river were somehow at fault for its overuse."

During the 21st century, drought took hold and continues to prevail in the Colorado River system. The river only produced normal or above-average runoff in 2005, 2008 and 2011, and major reservoirs in the basin dropped to historic lows. 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" by 2021 if current drying trends and water usage rates continue.

In late 2010, Lake Mead dropped to just 7 feet (2.1 m) above the first "drought trigger" elevation, or the level at which the lower basin states would begin to face water-supply cuts. 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 complicates 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

Further information: [[:List of flora of the Lower Colorado River Valley]]

The Colorado River and its tributaries often nourish extensive corridors of riparian growth as they flow through the arid desert regions of the watershed. Although riparian zones represent a relatively small area and water resources development has caused environmental degradation 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. 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.

Heavily forested banks of the Colorado River near Topock, Arizona

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, comprised 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.

Recreation

A rafting party on the Colorado River

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 run 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". 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 extremely popular summer destinations. Houseboating and water-skiing are popular 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 San Juan River near Mexican Hat, Utah

The Green River at Mineral Bottom, just north of Canyonlands National Park

Main article: List of tributaries of the Colorado River

The Colorado is joined by over twenty-five significant tributaries, of which the Green River is the largest by both length and discharge. The Gila River is the second longest and drains a greater area, but even historically, it averaged less than a third of the Green's flow. Both the Gunnison and the San Juan Rivers contribute more water than the Gila did naturally.

Statistics of the Colorado's longest tributaries
Name	State	Length		Watershed		Discharge
		mi	km	mi2	km2	cfs	m3/s
Green River	UT	730	1,170	48,100	125,000	6,048	171.3
Gila River	AZ	649	1,044	58,200	151,000	247	7.0
San Juan River	UT	383	616	24,600	64,000	2,192	62.1
Little Colorado River	AZ	356	573	26,500	69,000	424	12.0
Dolores River	UT	229	369	4,574	11,850	633	17.9
Gunnison River	CO	164	264	7,930	20,500	2,570	73
Virgin River	NV	162	261	13,020	33,700	239	6.8

See also

• Colorado Desert
• List of Colorado River rapids and features
• List of longest main-stem rivers in the United States
• List of longest rivers of Mexico
• Moab Tailings
• New London Bridge
• Upper Colorado River Endangered Fish Recovery Program

Works cited

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• Armstead, Lloyd Dean (1997). Whitewater Rafting in North America: More than 100 River Adventures in the United States and Canada. Globe Pequot. ISBN 0-7627-0083-1.
• What Every American Should Know About American History: 225 Events That Shaped The Nation. Adams Media. 2008. ISBN 1-59869-428-6. {{cite book}}: Cite uses deprecated parameter |authors= (help)
• Barnes, Will Croft (1988). Arizona Place Names. University of Arizona Press. ISBN 0-8165-1074-1.
• Benke, Arthur C.; Cushing, Colbert E. (2005). Rivers of North America. Academic Press. ISBN 0-12-088253-1.
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• Brown, Robert Leaman (1972). Colorado Ghost Towns – Past and Present. Caxton Press. ISBN 0-87004-218-1.
• Casey, Robert L. (2007). Journey to the High Southwest: A Traveler's Guide to Santa Fe and the Four Corners of Arizona, Colorado, New Mexico, and Utah. Globe Pequot. ISBN 0-7627-4064-7.
• Dellenbaugh, Frederick Samuel (1909). The Romance of the Colorado River: The Story of its Discovery in 1540, With an Account of the Later Explorations, and with Special Reference to the Voyages of Powell Through The Line of the Great Canyons. G.P. Putnam's Sons.
• Dolnick, Edward (2002). Down the Great Unknown: John Wesley Powell's Journey of Discovery and Tragedy Through the Grand Canyon. HarperCollins. ISBN 0-06-095586-4.
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• Fradkin, Philip L. (1996). A river no more: The Colorado River and the West (2 ed.). University of California Press. ISBN 0520205642.
• Touring Nevada: A Historic and Scenic Guide. University of Nevada Press. 1983. ISBN 0-87417-074-5. {{cite book}}: Cite uses deprecated parameter |authors= (help)
• Gudde, Erwin G.; Bright, William (2004). California Place Names: The Origin and Etymology of Current Geographical Names. University of California Press. ISBN 0-520-24217-3.
• Gupta, Avijit (2007). Large Rivers: Geomorphology and Management. John Wiley and Sons. ISBN 0-470-84987-8.
• Ghiglieri, Michael P. (2001). Over the Edge: Death in Grand Canyon (1st, tenth revision ed.). Flagstaff: Puma Press. ISBN 0-9700973-1-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Hutchinson, Robert (1995). Grand Canyon National Park: A Photographic Natural History. BrownTrout Publishers. ISBN 1-56313-611-2.
• Lankford, Andrea (2010). Ranger Confidential: Living, Working, and Dying in the National Parks. Globe Pequot. ISBN 0-7627-5263-7.
• Leuchtenburg, William Edward (2000). American places: encounters with history. Oxford University Press. ISBN 0-19-513026-X.
• Lindberg, Eric (2009). Colorado Off the Beaten Path: A Guide to Unique Places. Globe Pequot. ISBN 0-7627-5024-3.
• Lingenfelter, Richard E. (1978). Steamboats on the Colorado River, 1852–1916. University of Arizona Press. ISBN 0-8165-0650-7.
• Birds of the Salton Sea: status, biogeography and ecology. University of California Press. 2003. ISBN 0-520-23593-2. {{cite book}}: Cite uses deprecated parameter |authors= (help)
• 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.
• 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.
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• The Colorado River: Origin and Evolution. Grand Canyon Association. 2001. ISBN 0-938216-79-1. {{cite book}}: |access-date= requires |url= (help); Cite uses deprecated parameter |authors= (help)

External links

• 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