Colorado River

• 1. Introduction

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  Every drop of water in the Colorado River is used an average of 17 times in a single year. That statistic describes a river unlike almost any other. The Colorado River runs about 1,450 miles through the American Southwest and northern Mexico, which makes it the fifth longest river in the United States. Its name is Spanish for colored reddish, a reference to the heavy silt it carries. Its watershed reaches into seven U.S. states and two Mexican states, and it is often called America's Nile. About 40 million people depend on its water. The Southern Nevada Water Authority has described it as one of the most controlled, controversial and litigated rivers in the world. How does a river of canyons and whitewater turn into a plumbing system? What forces carved its famous gorges, and who lived beside it first? And what happens when the promises made about its water turn out to be larger than the river itself?

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• 2. Source To Sea

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  The Colorado begins in Rocky Mountain National Park, where it makes a short run south before turning west below Grand Lake, the largest natural lake in the state of Colorado. For its first 250 miles, it carves through the Western Slope, a sparsely populated region west of the Continental Divide. Many small streams feed it, along with larger ones such as the Blue, Eagle and Roaring Fork rivers. Past De Beque Canyon it spills out of the Rockies into the Grand Valley, a major farming and ranching region. At Grand Junction it meets the Gunnison River, one of its largest tributaries.

  Below Grand Junction, the river cuts across the Colorado Plateau, a vast high desert centered on the Four Corners. The climate turns far drier than in the Rockies, and the river sinks into gorges of bare rock, first Ruby Canyon and then Westwater Canyon as it crosses into Utah. Near Moab it slips out through The Portal, a gap between two sandstone cliffs about 1,000 feet high. Utah's slickrock country, with its narrow canyons and tilted folds of sedimentary rock, ranks among the most inaccessible places in the continental United States. In Canyonlands National Park the Green River, the Colorado's largest tributary, joins in, and the combined stream plunges into Cataract Canyon, named for its dangerous rapids. Next comes Glen Canyon, known for its arches and sculpted formations of Navajo sandstone, where the San Juan River arrives from the east.

  In Arizona the river passes Lee's Ferry and enters Marble Canyon, the beginning of the Grand Canyon, flowing beneath the Navajo Bridges. Below the Little Colorado River it swings west into Granite Gorge, the most dramatic stretch, where it has cut as much as a mile into the plateau. The walls expose some of the oldest visible rocks on Earth, dating back as far as 2 billion years. The 277 miles of river inside the Grand Canyon are known for hard whitewater, broken up by pools as deep as 110 feet. At the canyon's lower end the river widens into Lake Mead, held back by Hoover Dam and the largest reservoir in the continental United States. Farther downstream, the Mike O'Callaghan-Pat Tillman Memorial Bridge rises nearly 900 feet above the water, the highest concrete arch bridge in the Western Hemisphere.

  Below the dam, the Colorado enters the Lower Colorado River Valley, a desert region that depends on irrigation and tourism. Joseph C. Ives surveyed this stretch in 1861 and reported that the channel, banks, islands and bars shifted so continually that a description from one trip could be wrong within a week, or even a day. That restless river has since been confined. At Imperial Dam, more than 90 percent of the flow is moved into canals, including the All-American Canal, which irrigates California's Imperial Valley, the most productive winter farming region in the United States. At Morelos Dam, the entire remaining flow is diverted to irrigate Mexico's Mexicali Valley.

  Beyond San Luis Río Colorado the river lies wholly within Mexico, and since 1960 the stretch down to the Gulf of California has been dry or a trickle of irrigation runoff. The delta there is a floodplain of about 3,000 square miles. Before the early 20th century, anywhere from 10 to 20 million acre-feet of water a year reached it. An acre-foot is enough water to cover an acre of land a foot deep. In 1984, a rare run of wet years sent 16.5 million acre-feet of excess runoff all the way to the sea. Today the International Boundary and Water Commission occasionally allows a springtime pulse flow to recharge the delta.

  On the 18th of July 1746, a Croatian missionary in Spanish service, Father Ferdinand Konščak, made the first historical record of a tidal bore in the river's estuary. Locals called the wave El Burro. During spring tides it formed near Montague Island in Baja California and rolled upstream. It belonged to the era before 20th-century development drained the lower river.

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• 3. Deep Time

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  About 100 million years ago, in the Cretaceous period, much of western North America still lay beneath the Pacific Ocean. Between 50 and 75 million years ago, the Farallon Plate collided with the North American Plate. That collision, known as the Laramide orogeny, pushed up the Rocky Mountains. The Colorado first took shape as a west-flowing stream draining the southwestern part of the new range. The same uplift bent the Green River, once a tributary of the Mississippi River, westward toward the Colorado. Volcanic activity tied to the mountain building peaked 30 to 20 million years ago in the Mid-Tertiary ignimbrite flare-up. It built smaller formations such as Arizona's Chiricahua Mountains and buried the watershed under volcanic ash and debris.

  The Colorado Plateau began rising in the Eocene, between about 55 and 34 million years ago, and reached its present height around 5 million years ago. In that early era the river did not run to the Gulf of California at all. Before faulting opened the Gulf between 12 and 5 million years ago, the Colorado flowed west to the Pacific, possibly at Monterey Bay. It may even have helped form the Monterey submarine canyon. Crustal stretching in the Basin and Range Province began about 20 million years ago. The rise of the modern Sierra Nevada, starting about 10 million years ago, eventually turned the river south toward the Gulf.

  As the plateau kept climbing between 5 and 2.5 million years ago, the river held its ancestral course and sliced down into the rising rock, beginning the Grand Canyon. Geologists call a stream that behaves this way antecedent. The Dolores River's cut through Paradox Valley in Colorado shows the same pattern, as does the Green River's passage through Utah's Uinta Mountains. The exact timing and sequence of the canyon's formation remain uncertain.

  Between 1.8 million and 10,000 years ago, basalt flows from the Uinkaret volcanic field in northern Arizona repeatedly blocked the river inside the Grand Canyon. At least 13 lava dams formed. The largest stood more than 2,300 feet high and backed the river up for nearly 500 miles, all the way to present-day Moab, Utah. The lakes behind the dams left almost no sediment, which suggests that most of the dams lasted only a few decades. When erosion, leaks and cavitation broke them, catastrophic floods followed that may rank among the largest in North America, rivaling the Missoula Floods of the Pacific Northwest. Flood deposits show crests as high as 700 feet moving through the canyon, with peak flows as great as 17 million cubic feet per second.

  Sediment carried off the plateau built a vast delta, more than 10,000 cubic miles of material, which walled off the northern end of the Gulf in roughly a million years. Cut off from the ocean, that northern basin evaporated and left the Salton Sink, which reached about 260 feet below sea level. At least three times since, the river has changed course into the sink and turned it into Lake Cahuilla, which at its largest flooded up the valley to present-day Indio, California. Each time the Colorado returned to the Gulf, the lake needed about 50 years to evaporate. The present-day Salton Sea can be seen as the latest, much smaller version of that lake. In the 20th century the sink would fill once more, this time through a breach in a canal.

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• 4. First Peoples

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  Native Americans have lived in the Colorado River basin for at least 8,000 years. Evidence of agriculture, masonry dwellings and petroglyphs begins with the Fremont culture. The Ancestral Puebloans, also called Anasazi or Hisatsinom, became established in the Four Corners region around 1000 CE. They built multi-story pueblos known as great houses, and they developed complex systems to distribute drinking and irrigation water at Chaco Canyon in northwestern New Mexico and Mesa Verde in southwest Colorado. Chaco Canyon supported between 6,000 and 15,000 people at its height. Notably, the first major farming societies arose a good distance from the river itself.

  The Hohokam, present in the Phoenix area since about 0 CE, grew rapidly around 600-700 CE as they dug a large network of irrigation canals fed by the Salt River. At their peak they may have numbered anywhere from 30,000 to 200,000 people. Puebloan and Hohokam settlements were abruptly abandoned in the 1400s. Overuse of resources such as timber played a part, and so did severe drought that made it impossible to keep the irrigation systems running. Many Puebloans moved east to the Rio Grande Valley, and their descendants include the Hopi, Zuni, Laguna and Acoma peoples. The Akimel O'odham, also called the Pima, and the Maricopa are believed to descend from the Hohokam.

  The lower river belonged to the Patayan cultures, many of whom spoke Yuman-Cochimi languages. The Walapai, Havasupai and Yavapai lived in the Grand Canyon region. The Mohave, Halchidhoma, Quechan and Halyikwamai lived along the river between Black Canyon and the Mexican border, while the Cocopah lived around the delta. These river peoples relied on fishing and floodplain farming rather than irrigation, and mostly did not build permanent settlements. The site of modern Yuma has served as a river crossing since ancient times. The channel there is much narrower than the swampy bottomlands to the north and south, which opened trade with the Pima and Maricopa to the east and with coastal California tribes to the west.

  The Navajo, who call themselves the Diné, began moving into the basin around 1000-1500 CE. They were originally hunter-gatherers, but they learned farming from the Puebloans and gradually settled down, using irrigation extensively. Navajo Mountain and Rainbow Bridge in the Glen Canyon area took on deep religious meaning. The nearby meeting of the Colorado and San Juan rivers is regarded as the birthplace of clouds and rain.

  The Ute settled on the Colorado Plateau around 1500 CE, though they had lived farther north, in present-day Wyoming and northern Colorado, since at least 0 CE. They are the first known inhabitants of that part of the Rocky Mountains. Separate bands shared a common language and customs. The Uncompahgre, also called the Tabeguache, lived around the meeting of the upper Colorado and Gunnison rivers, an area that includes the Grand Mesa. The Weenuchiu lived along the San Juan River, and the Parianuche and Yamparika lived in the Yampa, White and Duchesne valleys. One Ute story tells of a battle with the Arapaho at Grand Lake, and the Ute believe the lake still holds the spirits of the dead.

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• 5. Explorers And Names

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  In 1536 Francisco de Ulloa may have been the first European to see the river, when he sailed to the head of the Gulf of California. In 1540 García López de Cárdenas became the first European to see the Grand Canyon, during Coronado's search for the Seven Cities of Gold, known as Cibola. Cárdenas was apparently unimpressed. He badly underestimated the canyon's size and left disappointed, with no gold. That same year Melchior Díaz explored the delta and named it Rio del Tizon, meaning fire brand river, after watching local people warm themselves in a distinctive way.

  By the late 1500s or early 1600s the Ute had acquired horses from the Spanish, and the animals soon spread among the Ute and Navajo for hunting, trade and warfare. That gave them an edge over the Goshutes and Southern Paiutes, who were slower to adopt them. The Navajo also took up herding after obtaining sheep and goats. In 1774 Juan Bautista de Anza became the first Spaniard to reach Yuma Crossing, befriended the Quechan and opened the Anza trail between Arizona and the California coast. Spain then founded two missions on the lower river. But Spanish efforts to control the crossing set off the 1781 Yuma revolt, in which over 100 soldiers and colonists were killed and the settlements were abandoned. The Quechan then kept foreigners away from the crossing until American mountain men and fur trappers arrived in the 1820s.

  The name Rio Colorado first appears in 1701, on missionary Eusebio Kino's map Paso por Tierra a la California. Kino also determined in those years that Baja California was a peninsula and not an island. Meanwhile, many Spanish and American explorers believed in a Buenaventura River running from the Rocky Mountains to the Pacific. In 1776 Silvestre Vélez de Escalante attached that name to the upper Green River, and later maps linked it to Lake Timpanogos, now Utah Lake, and on to California. The Dominguez-Escalante expedition reached the Colorado near the junction with the Dolores River and called the larger stream Rio San Rafael. They later forded it at the Crossing of the Fathers, which now lies beneath Lake Powell. John C. Frémont ended the myth in 1843, when he explored the Great Basin and found that no Buenaventura flowed west to California.

  American trappers on the upper Green River in Wyoming, who knew it as the Seedskeedee or a variant of that name, guessed in the 1820s that it joined the Colorado of the Spanish. William H. Ashley made an unsuccessful attempt to float from the Green River to the Colorado's mouth in 1825. In 1826 Jedediah Smith reached the lower Colorado, still calling it the Seedskeedee, and followed it upstream as far as Black Canyon. During the 1830s trappers from Wyoming worked their way down to Cataract Canyon and Glen Canyon, but none managed to navigate the full length of the river.

  Fort Yuma, founded by the U.S. Army in 1848, became the first permanent American settlement on the river. Hauling supplies overland was grueling, so the schooner Invincible tried to carry them upriver, but the delta's strong tides defeated it. Steamboats arrived in 1852, beginning with the sidewheeler Uncle Sam, whose first voyage from the Gulf to Yuma took fifteen days. In 1857 George A. Johnson steamed the General Jesup to Pyramid Canyon, over 300 miles north of Fort Yuma. Lieutenant Joseph Christmas Ives followed in a specially built shallow-draft steamboat, the Explorer, and reached Black Canyon, where Hoover Dam stands today. Ives had been sent to judge the river as a route for navigation, and he wrote: "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."

  The Grand Canyon itself was the last stretch to be surveyed. In 1869 John Wesley Powell set out with nine men from Green River Station, Wyoming. They were the first non-native party to travel the length of the Grand Canyon, and the first to travel by boat from the upper Green River to the lower Colorado. Powell led a second expedition in 1871 with funding from the U.S. government, and he kept up geographical and botanical surveys of the region until the 1890s. A different expedition, led by Robert Brewster Stanton in 1889-1890, surveyed a route for a railroad through the canyon that was never built.

  For decades the river's own name was unsettled. Fur trappers came to call the stretch above the Green River confluence at Cataract Canyon the Grand River, though nobody knows exactly why. Above the Gunnison River it was also called the Bunkara River, the Blue River or the North Fork of the Grand River until the 1870s. By the early 1900s the Grand River name covered the whole stream up to Grand Lake. Congressman Edward T. Taylor of Colorado petitioned a congressional committee for more than a decade to rename it the Colorado. Representatives from Wyoming and Utah, and the United States Geological Survey, objected that the Green River was longer and drained a larger area. Taylor countered that the Grand River carried more water, and he reportedly felt "slighted" that the Colorado did not begin in Colorado. On the 25th of July 1921, President Warren G. Harding signed House Joint Resolution 32, and the Grand River became the Colorado. The old name survives in Grand County and Grand Junction.

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• 6. Frontier Pressure

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  In 1858 gold turned up on the Gila River east of Yuma, and then along the Colorado at El Dorado Canyon, Nevada, and La Paz, Arizona. Prospectors and settlers clashed with the Mohave, which prompted U.S. Army expeditions that ended the Mohave War with the 1859 Battle of the Colorado River. In the 1870s the Mohave were moved to the Fort Mohave and Colorado River reservations. The Chemehuevi, and later some Hopi and Navajo, were also relocated to the Colorado River reservation, where together they form the Colorado River Indian Tribes.

  In 1864 General James Henry Carleton set out to expel the Navajo from the Four Corners region and enlisted the mountain man Kit Carson to lead the campaign. Carson, aided by the Navajo's Ute enemies, captured more than 8,000 Navajo and marched them by force to Fort Sumner, New Mexico. Hundreds died on what is now called the Long Walk and in the appalling conditions at Fort Sumner. After the Army failed to keep the reservation there running, the Treaty of Bosque Redondo created the Navajo Nation in the Four Corners, and the Navajo were allowed to return in 1868.

  A strike on the Blue River in 1859 led to the founding of Breckenridge, Colorado, and began the rush for gold and silver in the Upper Basin. Until the 1860s, southwestern Colorado had stayed largely untouched by westward expansion, because the United States had recognized Ute sovereignty by treaty. After Colorado Territory was carved out in 1861 and more mineral strikes followed, including at Ouray and Telluride, Ute leaders were pressured into the 1873 Brunot Agreement, which cost them most of their land. By 1881 the Army had driven out the last pockets of Ute resistance on the Western Slope, and the town of Grand Junction was incorporated a year later. The Denver and Rio Grande Western Railroad reached it by 1883 and finished a spur to Glenwood Springs in 1887.

  In Arizona and Utah territories, many early settlers were Mormons fleeing religious persecution in the Midwest. They founded farming colonies at Fort Santa Clara in 1855 and at St. Thomas in 1865. St. Thomas now lies beneath Lake Mead. In 1879 a group of Mormon settlers blasted the precarious Hole in the Rock Trail to cross the Colorado at Glen Canyon and went on to found the community of Bluff. In central Arizona, settlers uncovered canals that the Hohokam had dug centuries before and put them back in use.

  A Mormon militia that included John D. Lee carried out the 1857 Mountain Meadows Massacre, in which 120 non-Mormon settlers were killed. Fearing retribution, Lee moved in 1870 to the remote Pahreah Crossing in Arizona and took over a ferry that Jacob Hamblin had set up in 1864. It was the only river crossing for hundreds of miles that was not hemmed in by vertical canyon walls, and it became known as Lee's Ferry. Lee was tried and executed in 1877, yet the ferry stayed a major transportation link until the Navajo Bridge was finished nearby in 1928.

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• 7. Taming The River

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  In 1900 the California Development Company set out to irrigate the Imperial Valley, a dry basin on the California-Mexico border. Because the valley lies below sea level, river water could flow there entirely by gravity. Engineer George Chaffey designed the Alamo Canal, which left the Colorado near Pilot Knob, California, and ran south into Mexico. There it joined the Alamo River, a dry channel that had once carried flood overflow into the Salton Sink. By 1903 about four thousand people lived in the valley, and more than 100,000 acres had been developed.

  The canal was plagued by the Colorado's heavy sediment and swinging water levels. In low flows the river dropped below the intake, and in high flows silt clogged it, so crews had to keep digging new cuts. Early in 1905, flooding destroyed the intake gates. By August the breach had grown wide enough to swallow the river's entire flow, which poured into the bottom of the valley. The Southern Pacific Railroad tried to dam the flow to protect its tracks, but repeated floods hampered the work. It took seven attempts, more than 3 million dollars and two years for the railroad, the company and the federal government to close the breach. By then a lake 45 miles long covered part of the valley. The disaster showed that any further development would need a dam to control the river.

  In 1928 Congress authorized the Reclamation Service, today's U.S. Bureau of Reclamation, to build the Boulder Canyon Project. Its key feature was a dam in Black Canyon, 30 miles southeast of Las Vegas, Nevada. Hoover Dam was completed on the 30th of September 1935. Its reservoir, Lake Mead, can hold more than two years of the Colorado's flow. Hoover was the tallest dam in the world when it was built, and it housed the world's largest hydroelectric power plant. It also steadied the lower channel, trapped sediment, held back floods and stored water for dry years.

  The same act authorized the All-American Canal, a permanent replacement for the Alamo Canal that stays entirely inside the United States. Its intake sits at Imperial Dam, 20 miles above Yuma, Arizona. With a capacity of over 26,000 cubic feet per second, it is the largest irrigation canal in the world. The Imperial Irrigation District supplies water to 520,000 acres south of the Salton Sea, and the Coachella Canal waters another 78,000 acres. The hot, sunny climate allows year-round growing, so the valley has become one of the most productive farming regions in North America.

  Arizona once sent members of its National Guard to stop work on Parker Dam. The dam was built to feed the Colorado River Aqueduct, planned by the Metropolitan Water District of Southern California to supply Los Angeles. Arizona feared California would take too much water. The standoff ended in a compromise: Arizona dropped its objections, and the Bureau of Reclamation built the Gila Project, which irrigates 110,000 acres on the Arizona side. By 1941 the 241-mile aqueduct was delivering 1.2 million acre-feet a year to Southern California, and it now serves about 10 million people. The San Diego Aqueduct, opened in stages between 1954 and 1971, brings water to 3 million more.

  Far upstream, the Grand Ditch, finished in 1890 near La Poudre Pass at the river's headwaters, sent runoff across the Continental Divide to arid eastern Colorado. It was considered an engineering marvel and was the first of twenty-four transmountain diversions. About 80 percent of Colorado's people now live on the eastern slope, while 80 percent of the precipitation falls on the western slope. The Colorado-Big Thompson Project, begun in the 1930s, is the largest diversion and delivers 230,000 acre-feet a year to cities north of Denver. Together the diversions remove about 580,000 acre-feet a year from the basin, and Denver Water gets about half its supply from it.

  In 1956 Congress authorized the Colorado River Storage Project, whose first blueprints included two dams on the Green River in Echo Park Canyon, inside Dinosaur National Monument. The National Park Service and groups such as the Sierra Club objected, the fight drew national attention, and the Bureau dropped the idea in exchange for a larger dam at Glen Canyon. Glen Canyon Dam was built from 1956 to 1966. Few Americans knew the canyon existed, so opposition grew only after construction was well underway. David Brower, a Sierra Club leader, fought the dam during construction and for years afterward until his death in 2000. He believed he was personally responsible for failing to stop it, and he called it his "greatest mistake, greatest sin."

  In January 1964 the Bureau of Reclamation published the Pacific Southwest Water Plan, which proposed diverting water from the northwestern United States into the Colorado basin at an estimated cost of 3.1 billion dollars. Its Arizona centerpiece was a canal system, which became the Central Arizona Project, to carry Colorado water to Phoenix and Tucson. Central Arizona then depended on local supplies such as the 1911 Theodore Roosevelt Dam, and groundwater was being pumped at unsustainable rates while aquifers collapsed. To power the pumps, planners proposed two hydroelectric dams in the Grand Canyon, Bridge Canyon Dam and Marble Canyon Dam. Public pressure against them was immense with the Glen Canyon fight still fresh. Both were left out of the project's final authorization in 1968, and the boundaries of Grand Canyon National Park were redrawn to block future dams.

  The Navajo Generating Station, a coal-fired plant near Page, Arizona, supplied the pumping power in 1976 and ceased operation in 2019. The canal itself was built in stages between 1973 and 1993. It runs 336 miles from Parker Dam to Tucson, delivers 1.4 million acre-feet a year, irrigates 830,000 acres and supplies about 5 million people. Most of the plan's other facilities were never built, which left Arizona and Nevada exposed to future water reductions under the Compact.

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• 8. Dividing The Water

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  In 1922 six U.S. states signed the Colorado River Compact, which split the river's flow into two halves. The Upper Basin is the drainage above Lee's Ferry, covering parts of Colorado, New Mexico, Utah and Wyoming and a small portion of Arizona. The Lower Basin holds Arizona, California, Nevada and parts of New Mexico and Utah. Each half received 7.5 million acre-feet a year, a figure believed to be half the river's annual flow at Lee's Ferry. The deal assumed that about 17.5 million acre-feet passed that point every year. Yet the Upper Basin covers only 45 percent of the basin's land while supplying 92 percent of its runoff, and up to 90 percent of the river's flow starts as mountain snowmelt.

  Arizona initially refused to ratify the compact, fearing that California would claim too much of the Lower Basin's share. A compromise in 1944 gave Arizona 2.8 million acre-feet but ranked California's 4.4 million ahead of it in drought years. That year a treaty also promised Mexico 1.5 million acre-feet annually, and Morelos Dam was built in 1950 so that Mexico could use its share. This and nine other decisions, compacts, federal acts and agreements made between 1922 and 1973 form what is called the Law of the River. Under it, the Upper Basin must deliver at least 7.5 million acre-feet a year past Lee's Ferry, measured on a rolling 10-year average. The flow gauge there, 16 miles below Glen Canyon Dam, determines the allocations for the whole basin.

  Nevada received the smallest allocation of any state, just 0.3 million acre-feet. Officials there believed groundwater in the southern part of the state would cover future growth, and they cared more about securing a large share of the dam's electricity. By 1937 Las Vegas had tapped a pipeline into Lake Mead. In 2018 the city finished a second pipeline with a lower intake because the lake's level was falling.

  Native American water rights were largely ignored while the river was developed in the 19th and 20th centuries. When Parker Dam was finished in 1938, its reservoir flooded the riverside lands of the Chemehuevi. Ten tribes in the basin now hold or still claim rights to the Colorado's water. The first federally funded irrigation project in the United States was a canal built on the Colorado River Indian Reservation in 1867. The Navajo Indian Irrigation Project, authorized in 1962, serves part of the Navajo Nation in north-central New Mexico. About 40 percent of the Navajo reservation's residents must still haul water by truck over many miles. In the 21st century the Navajo have filed legal claims against Arizona, New Mexico and Utah, and a 2004 settlement gave them a 326,000 acre-foot allotment from New Mexico.

  The compact rested on barely 30 years of streamflow records. Tree-ring studies later showed that those decades were probably the wettest in the past 500 to 1,200 years. The natural long-term flow past Lee's Ferry is probably closer to 13.5 million acre-feet a year, and the unimpaired runoff there is estimated at 14.7 million acre-feet for the period 1906-2023. More water has therefore been promised to users than the river actually holds.

  Drought has made the shortfall worse. The southwestern North American megadrought, the most severe on record, began in the early 21st century, and the basin produced above-average runoff in only five years between 2000 and 2021. The region is warming, with earlier snowmelt, lower precipitation and greater evaporation from land and plants. A 2004 study found that a 1-6 percent drop in precipitation could cut runoff by as much as 18 percent by 2050. A dry spell from 2000 to 2004 left Lake Powell at just a third of capacity in 2005, its lowest level since it first filled in 1969.

  In late 2010 Lake Mead neared the drought trigger elevation of 1,075 feet, below which supplies to Arizona and Nevada would be cut. Their water rights are junior to California's, so their allocations can legally fall to zero before California loses any. A wet winter in 2011 brought brief relief, but dry conditions returned. In 2014 the Bureau of Reclamation cut releases from Lake Powell by 10 percent, the first such cut since the 1960s, and Lake Mead dropped to its lowest level since 1937. After two more extremely dry winters, Lake Powell fell in July 2021 below the record low it had set in 2005.

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