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The River That Flowed Uphill

Carving the Canyon

Submitted by bog on Sat, 07/07/2012 - 17:19

There's little doubt that the Colorado River carved the Grand Canyon. But exactly how it did so is still a mystery. Water flows downhill, so when a river encounters rising terrain it is diverted, always seeking the path of least resistance to reach the sea.

So why does the course of the Colorado River cut through the Kaibab and Coconino plateaus almost at their highest point? As the broader Colorado Plateau rose, lifted by the same mountain-building forces that raised the Rocky Mountains, the Colorado River should have gone somewhere else.

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Headward Erosion

Submitted by bog on Sat, 07/07/2012 - 17:19

A recent theory suggests that the Colorado River achieved its present course by a combination of headward erosion, stream capture, and lake basin overflow, in the process linking ancient canyons and creating new ones to form the Grand Canyon. Evidence suggests that the lower Colorado River below the Grand Canyon, which now forms the border between Arizona and Nevada and California, found its present course by the overflow and spilling of ancient lake basins. These lakes formed as crustal stretching created the parallel mountains nad valleys of the Great Basin.

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Stream Capture

Submitted by bog on Sat, 07/07/2012 - 17:20

At the center of this map, Seventyfive Mile Canyon, on the left, is about to capture Tanner Canyon, on the right

Because water rushing down the steep slopes at the upper ends of the canyons has far more erosive power than the gentler flows in the lower canyons, erosion proceeds rapidly headward into the surrounding plateau. If the headward-eroding canyon rim encounters another, lower-gradient stream that can't deepen its bed as rapidly, it will capture and divert the headwaters of the lower-gradient stream, further increasing the runoff and erosion in the steeper canyon.

If the headward-eroding canyon rim encounters another, lower-gradient stream that can't deepen its bed as rapidly, it will capture and divert the headwaters of the lower-gradient stream, further increasing the runoff and erosion in the steeper canyon.

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Seventyfive Mile Canyon

Submitted by bog on Sat, 07/07/2012 - 17:21
Looking across Tanner Canyon to the saddle at the head of Seventyfive Mile Canyon, seen just below and right of photo center

Looking across Tanner Canyon to the saddle at the head of Seventyfive Mile Canyon, seen just below and right of photo center

This process can be seen today in the eastern Grand Canyon where Seventyfive Mile Canyon is eroding away the saddle at its head. Very soon, geologically speaking, Seventyfive Mile Canyon will capture the drainage of Tanner Canyon, which is only a few dozen yards from the saddle, and divert upper Tanner Canyon into Seventyfile mile Canyon.

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Ancestral Upper Colorado River System

Submitted by bog on Sat, 07/07/2012 - 17:21

Stream capture may account for the fact that the upper Colorado River east of the Grand Canyon appears to have once flowed in the opposite direction, probably fed by the Little Colorado River. The Little Colorado most likely drained mountain highlands in eastern Arizona, ultimately draining into an inland lake in the vicinity of the present Rocky Mountains.

According to the stream capture theory, around 70 million years ago what is now the upper Colorado River east of the Grand Canyon actually flowed in the opposite direction, fed by the Little Colorado River. The Little Colorado drained mountain highlands in eastern Arizona, ultimately draining into an inland lake in the vicinity of the present Rocky Mountains.

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Capturing the Upper Colorado River

Submitted by bog on Sat, 07/07/2012 - 17:22

About 16 million years ago, the basin-and-range country of far western Arizona formed, causing drainage to increase from the rising plateaus in the Grand Canyon region. The increasing drainage filled the basins between the mountains, creating lakes that eventually overflowed, breaching the rim of the basin, and creating a lake in the next lower basin. A succession of these events created the modern course of the Colorado River along the borders of Arizona, Nevada, and California, downstream from the Grand Canyon. The increasing runoff also caused the ancestral lower Colorado River to erode headward to the east, ultimately capturing the drainages of older canyons. The portion of the Grand Canyon between Parashant Canyon and Peach Spring Wash has been dated at 70 million years, much older than the far western or eastern portions of the Canyon. This section appears to have been part of an ancient canyon system that drained to the north, into Utah. When the lower Colorado River captured this canyon, it reversed the flow and added its run off to its own. This process was probably repeated many times, and eventually, the headward-eroding lower Colorado River captured the ancestral upper Colorado River and the Little Colorado in the vicinity of the present confluence of the Little Colorado and Colorado rivers.

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Reversing the Colorado River

Submitted by bog on Sat, 07/07/2012 - 17:23

Backward tributaries in Marble Canyon

The Colorado River now flows southwest through ever deeper Marble Canyon, which is carved directly up the slope of the surrounding Marble Plateau. All the side canyons meet the river angling upstream. From the air or on a small-scale map, the effect is striking; the Colorado River is flowing the wrong way.

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Glacial Floods

Submitted by bog on Sat, 07/07/2012 - 17:23
Stream in flood

Stream in flood

The modern course of the Colorado River was established by about five to six million years ago and the Grand Canyon began to appear. Since then, successive glacial periods followed by melting repeatedly sent large volumes of water down the Colorado River and its main tributary, Green River, rapidly increasing its depth. Some geologists think the depth of the Grand Canyon was doubled in this way during the last two million years.

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