In 2006,

I traveled to Cambodia to write a story for National Geographic about Zeb Hogan, an American fish biologist who was doing conservation work on the Mekong River. Zeb had just launched what he called “The Megafishes Project,” aimed at finding, studying, and protecting the world’s largest freshwater fish, the 30 or so species that can grow at least six feet long or reach weights of more than 200 pounds, several of which are found in the Mekong. Our work together has continued since then, culminating in a book called Chasing Giants: In Search of the World’s Largest Freshwater Fish, which University of Nevada Press is publishing this month.

Giant freshwater fish, from gargantuan gars to sumo-sized stingrays, are among the most threatened animals on the planet, with many species facing possible extinction. What I learned is that they are also good indicators of river health, and so the book became as much a tale about the poor state of our world’s rivers as the increasingly rare and endangered fish that we find in them.

Some of these giant fish species live in American rivers. As it turns out, one of them once swam in the Colorado River. As most people are aware, the Colorado — on which 40 million people, including Las Vegas residents such as me, today depend — is drying up because of a combination of chronic overuse and a historic drought driven by climate change. In our book, we explore the history and evolution of the river that built the American West. An adapted excerpt follows.

The Nile Of North America

FEW RIVERS ON Earth have both changed the world through which they flow and had change brought upon them as much as the Colorado River. Following the creation of the Rocky Mountains, beginning 75 million years ago, the Colorado formed as a river flowing straight west. It was not until the rising Colorado Plateau reached its peak six million years ago that the river established its present course, running southwest from the Rockies, through what is now the Grand Canyon, forming a vast marshy delta as it flowed into the Gulf of California.

Of course, the Grand Canyon did not exist then. It was the powerful waters of the Colorado and its tributaries that cut through layers upon layers of rock to forge what is today one of the world’s greatest natural wonders. For millions of years, the Colorado kept flowing, its power unchallenged, until eventually humans set foot in the river, and nothing would ever be the same again.

It was long thought that the first humans settling on the Colorado Plateau — the desert area that today centers on the Four Corners region, where Colorado, Utah, New Mexico, and Arizona meet — were Paleo-Indians of the Clovis and Folsom cultures, who arrived there around 12,000 years ago. But recent discoveries of human settlements at the Eagle Rock excavation site, near the small town of Delta, Colorado, push that time line back at least 1,000 years.

Over the following millennia, human use of the Colorado River basin revolved primarily around hunting and fishing. But with time, various tribal societies began to garden, and they eventually developed large farms. Indigenous groups are known to have farmed large swaths of land along tributaries in present-day Arizona more than 1,000 years ago. Archaeologists have uncovered giant storage reservoirs and hundreds of miles of ancient canals around Phoenix, evidence of what may have been the largest irrigation system built by prehistoric peoples in North America, a remarkable feat considering canals would have been dug by hand using wood, bone, and stone tools.

But the construction of irrigation canals on some of the Colorado’s tributaries is also believed to have changed the morphology of those waterways, complicating agriculture. When a megadrought hit the region in the 14th century, entire societies soon collapsed. As some tribes moved out of the area, the Navajo, who had migrated from the north, established themselves as the dominant People in the river basin, continuing to farm the land.

The first European to discover the river, in 1539, is said to have been the Spanish explorer Francisco de Ulloa, though it is not clear if he entered the river from its mouth or even recognized that there was a major river running north. Instead, it may have been Hernando de Alarcón, another Spanish explorer, who became the first European to ascend the river, which was described at the time as running “with so great a rage into the land that it was a thing to be marvelled at.”

After the United States purchased the territory of the Colorado River basin from Mexico in 1848, the discovery of gold in California triggered a western migration in search of riches. The importance of the Colorado to the future settlement of the southwestern United States became increasingly clear. With water, anything was possible in the desert, and as agriculture intensified throughout the basin, scholars referred to the river as “the Nile of North America.” Like the Nile, the Colorado originated in the mountains, flowed through a hot desert, and carried with it substantial amounts of silt down to the sea.

But the expansion of agriculture required more than small-scale gravity irrigation systems. It demanded big dams. By the turn of the century, most dams that existed in the western United States were small private ventures, mostly constructed from earth and rock, diverting water for mining businesses or irrigation of personal properties. But technology emerged to build bigger concrete dams for larger-scale irrigation, water supply, and flood control, and a new federal program created by the Newlands Reclamation Act of 1902 paved the way for such dams to be built in the Colorado River basin. Within a few years the first one, the Laguna Diversion Dam, was completed. Located on the main stem of the Colorado, near the Mexican border, the dam diverted water to the farm fields of Arizona’s Yuma Valley. It also ended boat travel to the north. Meanwhile, a much bigger dam was being constructed northeast of Phoenix, on the Salt River, a Colorado tributary. The Theodore Roosevelt Dam would store water and provide flood control at a huge scale. When it opened, in 1911, it was the largest masonry dam in the world — 280 feet high — and for a while Roosevelt Lake, created by the dam, was the world’s largest artificial reservoir. The project cost a staggering $10 million, but was considered a huge economic and engineering success. Irrigating vast land areas, the Roosevelt Dam would contribute, more than any other dam, to the settlement of central Arizona.

California’s farmers also coveted the precious waters of the Colorado, which marks the state line between Arizona and California but never enters the state proper. From early on, California sought to bend the Colorado to its own benefits, sometimes causing great ecological upheaval. One example is California’s largest lake, the Salton Sea. It formed in 1905 when massive flooding caused the Colorado River to break through an irrigation canal headwork that had been built as part of a project to provide water to California’s Imperial Valley. For 18 months, water flowed freely into the Salton basin, before the levee break was filled and the Colorado River forced back into its channel.

The newly formed Salton Sea created a rich oasis for birds, and the later introduction of various game fish built up a lucrative sport fishery around the lake. But without a natural outlet, water trapped more than 200 feet below sea level evaporated over the years, leaving increasing levels of salt behind. The result has been an environmental disaster for birds, fish, and humans alike, as the shrinking Salton Sea has transformed into a salty dust bowl.

But it wasn’t just farms in California that needed water to grow in the early 1900s; cities did too, especially fast-expanding Los Angeles. With water supply from its eponymous river woefully inadequate to sustain L.A.’s unprecedented growth, planners in Southern California sought to harness water from greater Los Angeles, San Bernardino, and San Diego areas. To this day, it is one of the primary sources of drinking water for Southern California.

By this time, a dam construction boom was well under way in the United States, with new federally funded projects designed not only for human consumption and irrigation purposes but also for generating electricity. The biggest, of course, was the Boulder Dam, later renamed Hoover Dam, built on the border between Nevada and Arizona, upstream from Lake Havasu. Completed in 1936, Hoover Dam became both the highest and largest dam in the world, and once online it earned the title of the world’s largest hydroelectric facility.

For decades, building dams represented progress. Few, if any, considerations were given to things like fish. But by the 1950s, a chorus of voices had begun warning about the environmental consequences of the dam boom.

One project that came under particularly heavy criticism was the building of a huge dam at Glen Canyon near the town of Page in northern Arizona. Initial plans to build the dam in the 1920s had been scrapped, only to be resurrected some 30 years later. Critics questioned the dam’s economic justifications, as well as the environmental damage caused by building a dam across a gorge lined with sandstone, which would flood the scenic Glen Canyon and threaten the integrity of the Grand Canyon just downstream.

Supports far-flung places for their ever-growing desert cityscapes. In 1922, California was one of seven states to enter the Colorado River Compact, an agreement to provide for the “equitable division and apportionment” of the use of waters from the Colorado River system, allowing California to stake a greater claim to the Colorado.

Eventually, Southern California secured its supply of Colorado River water with the construction of Parker Dam on the California-Arizona border. Completed in 1938, the dam created Lake Havasu, from which water would be pumped through the new Colorado River Aqueduct running west across the Mojave Desert to the east side of the Santa Ana Mountains, supplying almost all cities in the

Still, the project went ahead, and in 1963 Glen Canyon Dam stood 583 feet above the Colorado River, the second largest dam in the U.S. after Hoover, and with a newly plugged Lake Powell next to it. By then, all resemblance to the historic Colorado River had long since faded, with every drop of water litigated and allocated. In time, the mighty river that had once carved out America’s most iconic landscape would no longer reach the sea.

The Best Fish

OF ALL THE major rivers in North America, the Colorado has always had the lowest diversity of native fish, particularly in the lower part of the basin (below the Glen Canyon Dam). Still, few others have seen their native fish fauna so severely decimated. Only eight fish species have evolved in the lower basin, and those belong to only two families: minnows and suckers, both of the Cypriniformes order. (In comparison, there are almost 300 native fish species in the Mississippi River.)

National Parks & Wildlife Refuges

90%

Of water originates in the upper basin

18 SPANS STATES7

40 MILLION PEOPLE SUPPLIES WATER TO : 1,450 MILES

At the same time, the Colorado River has the highest level of fish endemism of any river system in North America, with six of the lower basin’s eight native species found in no other river in the world.

Why is that? A lot of it likely has to do with the geographic isolation of the Colorado system, and also the harsh environmental conditions that existed before the river was transformed by humans. For sunfish, catfish, and other fish that evolved outside the basin, the Colorado’s turbid and unpredictable waters, running through muscular mountains and scorching deserts, were too much to handle.

Drought no doubt played a key role in the evolutionary adaptations of fish in the Colorado. The late Wendell L. Minckley, an ichthyologist at Arizona State University who had five fish species named after him, speculated that seasonal low flows, amplified by drought, caused fish to retreat from broader river channels into deeper and more secure canyon reaches. Stuck in those spaces, some species would be eaten by others, and generally only a few larger fish would survive.

One such survivor is the Colorado pikeminnow, which is believed to have evolved more than three million years ago. It is the only large fish found in the Colorado River system that, historically, reached lengths of up to six feet, and thus qualifies as a “megafish,” though this beautiful fish, with its long, streamlined body of olivegreen hue, no longer grows this big, nor can it be found anymore in the Lower Colorado River basin.

With its big fins, small eyes, and a large mouth that, despite it being a voracious carnivore, lacks what we normally call teeth, the Colorado pikeminnow is the largest of four pikeminnow species and is known for long-distance spawning migrations of more than 200 miles in late spring and early summer.

Like some other native Colorado fishes, such as the razorback sucker and bonytail, Colorado pikeminnows can live almost 50 years and produce tens of thousands of eggs each spawning season. This has allowed the pikeminnows to survive through prolonged periods of drought when spawning may have been impossible. Once favorable conditions return, they can repopulate the river in a single season, in contrast to many other freshwater fish that live fewer than 10 years and can produce only a few hundred offspring each year.

But the story of the Colorado pikeminnow is, like that of many other giant fish, ultimately one of decline. Naturally, this has occurred at the hands of humans, and we’ve managed to cause this in a trifecta of progressive harm.

From the time humans first arrived at the Colorado River, they likely targeted the pikeminnow for food. Native American fishing practices throughout time have been well documented. One nomadic people, the Chemehuevie, who populated Southern Nevada and western Arizona, called the Colorado pikeminnow Ah´chee Ah´had, or simply “the best fish.”

The Colorado pikeminnow was also a valued food source among early white settlers, who alternatively called it “squawfish” — a name commonly used until very recently, despite its blatantly derogatory basis — and “Colorado white salmon,” a reference to the pikeminnow’s migratory behavior. There are plenty of stories of pikeminnows weighing up to 100 pounds, with settler journals describing catches producing “quite a few meals for the family.”

Historical accounts from the turn of the 20th century tell of farmers diverting the river’s water into their orchards and, after the water dried up, hundreds of pikeminnow being scattered across the fields, from where they were pitchforked out and used as fertilizer. Still, the pikeminnow remained the Colorado River’s top predator in the early 1900s, and they were a popular target fish for anglers, known to take bait in the form of mice, birds, and even small rabbits, despite their “teeth” being found only on a bony circular structure located deep within their throats.

But people also wanted to fish other species, for both food and recreation, and this brings us to the second way we have caused the decline of the pikeminnow: by introducing nonnative fish species into the Colorado River.

Non-native species that take root in new ecosystems and, in many cases, cause trouble are known as invasive organisms. Worldwide, and throughout history, there is said to have been more than 8,000 introductions of fish species into river basins outside their native range, making freshwater fish among the most introduced animals in the world.

Stocking of nonnative fish in the Colorado probably began as early as1881, while the countryside was still being settled. By 1910, common carp, bullhead, and channel catfish were prevalent throughout the lower river. The new arrivals increased the competition for food among all aquatic inhabitants. But even as food themselves, the catfishes caused great harm to the pikeminnows that preyed on them. The pikeminnows would get their prey’s barbels stuck in their throat when they tried to swallow the catfish, causing them to suffocate and die.

Even so, the pikeminnow might have survived these first two threats had it not been for the third and most destructive way we have caused the decline of the species: by building dams. In the Colorado River system, the dams proved disastrous for the migratory pikeminnow; neither the reservoirs behind the dams nor the cold, clear water flowing from them provided suitable habitat for a species that had evolved in wildly different and more extreme environments.

The last Colorado pikeminnow below Glen Canyon Dam was recorded in 1975. The following year, the species became federally protected and listed as endangered. Efforts to reintroduce the pikeminnow to the lower basin, using brood stock from the former Dexter National Fish Hatchery and Technology Center in New Mexico, failed because the fish would not reproduce, and eventually the efforts were abandoned. It left the Lower Colorado with the dubious distinction of being among the few major rivers in the world with an almost entirely introduced fish fauna.

THE DAM’S END

BECAUSE IT’S HARD to find a river as exploited by humans as the Colorado, it may come as a surprise to some that it is also considered the birthplace of modern U.S. environmentalism. Water in the American West played an important role in the early goings-on of the country’s 20th-century conservation movement.