The Colorado River is the principal river of the southwestern United States and northwest Mexico. The 1,450-mile river flows from the Rocky Mountains to the Gulf of California, draining 246,000 square miles 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 much 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.
Heavily forested banks of the
Colorado River near Topock, Arizona
Wildlife and plants
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, which supports 358 species of birds despite the reduction in 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.
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 brown trout – were introduced throughout the 19th and 20th centuries, mainly for sport fishing purposes.