Colorado River - 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.

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, Utah. The lack of associated sediment deposits along this stretch of the Colorado River, which would have accumulated in the impounded lakes over time, suggests that most of these dams did not survive for more than a few decades before collapsing or being washed away. 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).

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