The Colorado, which drains the arid south-western quarter of the USA, is the world’s most formidable canyon-cutter. In response to the tilting uplift of the Colorado Plateau, the river has cut more than 1,000 miles of deep canyons, the most spectacular stretch of which is the 220 mile long Grand Canyon in northern Arizona. The Colorado River rises from snowmelt high in the Rocky Mountains in the northern Colorado, on the westward slopes of the continental divide. It flows southwest across the Colorado Plateau into Utah, where the confluence with the Green River in the Canyon lands region brings waters from the northernmost reaches of its drainage basin in Wyoming. Downstream in northern Arizona, the main trunk of the Colorado’s branching canyon system- the Grand Canyon- reaches 18 miles in width and cuts down through layers of sedimentary rock that record 2 billion years of geological history. Below the canyon after exiting the plateau, the course turns southwards, forming the California- Arizona state line.
Along its lower course the Colorado is a slow, meandering river, laden with fine silt and subject to flooding. The last 80 miles run through Mexico to a shrinking delta. The Colorado River is the most managed river in the world. Large-scale engineering began in the northern part of the drainage basin in the 1920s, with great tunnels cut beneath the Continental Divide to divert some of the river’s water eastwards to the Great Plains. Subsequent attention has focused on the lower course.
The 726ft high Hoover Dam created Lake Mead in 1935, and provides both electricity and irrigation water. Downstream, considerable amounts of water are diverted to the cities of Las Vegas, Phoenix, Tucson, San Diego, and Los Angeles. The Imperial Dam diverts most of the water that remains along the All-American Canal to California’s Imperial Valley. Although the flow at the mouth of the Colorado is now much reduced, it has in the past effected great changes upon the coastline. The Gulf of California once extended much farther north until the Colorado changed its course in prehistoric times. Sediment deposited on the river’s banks dammed the Gulf, creating the current shoreline and the great Salton Sink, a stretch of former sea bed that extents inland some 300 miles. In 1905 a breached irrigation channel allowed the Colorado’s waters to flood into the sink, creating the 70 mile long Salton Sea.
The climate of the Colorado Plateau is hot and arid. Only cacti and low desert shrubs can grow, there are no trees, even at the river’s edge. Created by the river, the magnificent rock cliffs of the Grand Canyon are preserved by the aridity of the desert climate. At the base of the Canyon, the river has cut down to 1.7 billion year old rocks. At night the ground cools, the temperature drops, and moisture condenses. The cold fog creates a katabatic wind by sinking into the valley, displacing warmer air, as here in the Grand Canyon, USA.
The 710ft high Glen Canyon Dam is located in Arizona about 110 miles upstream of the Grand Canyon. It was constructed in the 1960s to store water from the winter rains to generate electricity later in the year, when the river’s natural flow diminishes. By restricting the flow of the Colorado, the dam has had an alarming effect on the environment along riverbanks in the Grand Canyon. Instead of being scoured clean by regular winter floods, parts of the canyon are now starting to fill with sediment. In the 1990s, attempts were made to redress the balance by releasing artificial floods from the dam.