Grand Canyon: How Do You Get a River Over a Mountain?
December 03, 2012
English: The Colorado River near Nankoweap Creek and the granaries, Grand Canyon (Photo credit: Wikipedia)
By David Coppedge
One would think that the Grand Canyon, one of earth’s most prominent geological features, studied by geologists for 140 years, would be well understood. Wrong. “The Colorado River’s integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in the cutting of Grand Canyon and the region’s landscape evolution.” That’s how Joel Pederson (Utah State) began the cover article in GSA Today this month,1 a bimonthly journal of the Geological Society of America. The mystery he investigated is how the Colorado River ran over a mountain: the Kaibab uplift.
The Kaibab uplift is the broad
southern end of the Colorado Plateau through which the Grand Canyon
has been carved. Rivers do not normally flow uphill. The
upper Colorado River, roaring from Rocky Mountain snow melt, faces
this obstacle; yet here the Grand Canyon cuts right into the plateau
at a steep monocline that extends north-south through Utah, and
manages to run westward through the high province till emerging from
the Grand Wash Cliffs at the west edge of the plateau. There,
at modern Lake Mead, the river suddenly enters the Basin and Range
province of Nevada. It flows onward to the southwest, emerging
at the Gulf of California (a nice tour to take in Google Earth).
Of
the many theories to explain this phenomenon since John Wesley Powell
ran the river for the first time in the 1870s, three have survived.
(Powell’s own idea that the river cut downward as the plateau
raised upward did not last long, because of dating discrepancies: the
plateau was thought to rise much earlier than the river.) If
there had been an antecedent river, where is the huge delta that
should have formed at its terminus? His own study showed that
the Muddy Creek Formation (Google Earth, 36°42’45″ N, 114°19’40″
W) looks more like drainage from the ancestral Virgin River, not the
Colorado. If the early river had exited through the Little
Colorado, why is there no evidence at that location? The
least-likely explanation had been the “precocious gully” theory
of Charlie Hunt. Beginning in 1956, Hunt imagined a
southwestern river cutting headward into the plateau and joining up
with the ancestral Colorado river. “Hunt’s hypothesis (b),
that the river arrived in the central-western Grand Canyon area and
simply infiltrated and terminated, never gained traction—and was
not well loved even by Hunt himself,” Pederson said….
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