The River That Flowed Uphill
At the center of this map, Seventyfive Mile Canyon, on the left, is about to capture Tanner Canyon, on the right. Click on the map for a larger view.
Looking across Tanner Canyon to the saddle at the head of Seventyfive Mile Canyon, seen just below and right of photo center. Click on the image for a larger view.
Backward tributaries in Marble Canyon. Click on the map for a larger view.
Stream in flood. Click on the image for a larger view.
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.
Headward Erosion
One current theory suggests that the present Colorado River achieved its course by a combination of headward erosion and stream capture. 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.
Stream Capture
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.
Seventyfive Mile Canyon
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.
Ancestral Upper Colorado River System
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.
Capturing the Upper Colorado River
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. This caused the ancestral lower Colorado River to erode headward to the east, ultimately capturing the upper Colorado River in the vicinity of the present confluence of the Little Colorado and Colorado rivers.
Reversing the Colorado River
At the same time, the rise of the Rocky Mountains disrupted the northeast flow of the Colorado River and eventually caused the river to reverse its flow. 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.
Glacial Floods
The modern course of the Colorado River was established by about two to six million years ago. Since then, sucessive 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.
s