<P> Rifting started to create the Gulf of California far to the south 6 to 10 million years ago . Around the same time, the western edge of the Colorado Plateau may have sagged slightly . Both events changed the direction of many streams toward the sagging region and the increased gradient caused them to downcut much faster . From 5.5 million to 5 million years ago, headward erosion to the north and east consolidated these streams into one major river and associated tributary channels . This river, the ancestral Lower Colorado River, started to fill the northern arm of the gulf, which extended nearly to the site of Hoover Dam, with estuary deposits . </P> <P> At the same time, streams flowed from highlands in central Arizona north and across what is today the western Grand Canyon, possibly feeding a larger river . The mechanism by which the ancestral Lower Colorado River captured this drainage and the drainage from much of the rest of the Colorado Plateau is not known . Possible explanations include headward erosion or a broken natural dam of a lake or river . Whatever the cause, the Lower Colorado probably captured the landlocked Upper Colorado somewhere west of the Kaibab Uplift . The much larger drainage area and yet steeper stream gradient helped to further accelerate downcutting . </P> <P> Ice ages during the Pleistocene brought a cooler and wetter pluvial climate to the region starting 2 to 3 million years ago . The added precipitation increased runoff and the erosive ability of streams (especially from spring melt water and flash floods in summer). With a greatly increased flow volume the Colorado cut faster than ever before and started to quickly excavate the Grand Canyon 2 million years before present, almost reaching the modern depth by 1.2 million years ago . </P> <P> The resulting Grand Canyon of the Colorado River trends roughly east to west for 278 miles (447 km) between Lake Powell and Lake Mead . In that distance, the Colorado River drops 2,000 feet (610 m) and has excavated an estimated 1,000 cubic miles (4,200 km) of sediment to form the canyon . This part of the river bisects the 9,000 - foot (2,700 m) - high Kaibab Uplift and passes seven plateaus (the Kaibab, Kanab, and Shivwits plateaus bound the northern part of the canyon and the Coconino bounds the southern part). Each of these plateaus are bounded by north to south trending faults and monoclines created or reactivated during the Laramide orogeny . Streams flowing into the Colorado River have since exploited these faults to excavate their own tributary canyons, such as Bright Angel Canyon . </P>

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