The topographic map at the
right shows part of the Marble Canyon section of the Grand
Canyon. Contour lines equal 100 feet. The Colorado River flows
from NNE (Mile 11) to SSW (Mile 32). On the west side of the
river there are four major tributary canyons (and a couple
more minor ones) that flow from WSW to ENE. In north to south
order these are, Soap Creek, House Rock, North Canyon, and
South Canyon. These arroyos flow "against the grain" with
respect to the direction that the Colorado River is flowing.
The result gives a "barbed" appearance to these arroyo
canyons. What is the origin of these barbed tributaries?
If we measure the length of these arroyo canyons
(using a "start" marker where they first obtain a depth of 500
feet below the Kaibab Limestone surface), we note that South
Canyon (at the south end) is about 4 miles long. North Canyon
and House Rock are about 3 miles each, and Soap Creek (at the
north end) is the shortest at about 2 miles. If we assume that
the distance that headwall erosion can work back upstream is a
function of time, then these lengths are a measure of how long
headwall erosion has had a chance to work. Thus, South Canyon
(at the south end) is the oldest of the side canyons, and Soap
Creek Canyon at the north end is the youngest.
A second feature of note (in the upper left
corner of the topographic map) is the Paria Plateau, which
ends abruptly at the Vermilion Cliffs. These cliffs gradually
erode back northward with time. Thus, we are looking at their
present position. If we go back in time, they would be found
further south. The further back in time - the further south.
If we take a close look at the contours on the
map, we note that Soap Creek (the northernmost and YOUNGEST
"barbed" wash/canyon) occupies the CURRENT low ground between
the northward dipping Kaibab Limestone surface and the present
position of the Vermilion Cliffs. It is currently establishing
its drainage system in what is now the lowest topographical
area. Its arroyo is becoming entrenched, thus it will preserve
this drainage system even though the Vermilion Cliffs can be
expected to retreat further north with time.
House Rock Canyon/Arroyo is next as we proceed
southward. If we go back 200,000 years or so, the present
location of House Rock Arroyo occupied the lowest ground
between the north dipping Kaibab Limestone and the Vermilion
Cliffs. (The Vermilion Cliffs would extend further south
200,000 years ago, and Soap Creek didn't exist yet.) Since
then, the cliffs have retreated, but just as Soap Creek is
becoming imbedded in the present topography, House Rock Arroyo
became imbedded in the path it established 200,000 years ago.
Still further south we find North Canyon/Arroyo.
The Vermilion Cliffs are well north of here now, but if we go
back far enough in time (before House Rock and Soap Creek
existed), then North Canyon/Arroyo occupied the low ground.
After it had a large enough drainage area, it became
entrenched, and has maintained this entrenched drainage ever
Using the now familiar pattern, it is apparent
that South Canyon/Arroyo occupied the low ground if we go back
still further. We can now form a rule as to how the
With time, the Vermilion Cliffs retreat
northward. As the easily eroded lower layers (Chinle and
Moenkopi) wash away, slabs of the upper layers break off the
cliffs, fracture and are gradually removed by whatever
drainage is currently occupying the low ground between the
north dipping Kaibab Limestone and the retreating cliffs. As
the cliffs retreat, the surface area drained by the current
northernmost arroyo gradually increases. When this surface
area becomes large enough (A larger surface area generates
larger thunderstorm runoffs which generate larger eroding
power.), the current arroyo starts eroding down into Kaibab
surface. The arroyo is now entrenched, and can no longer
migrate sideways to keep pace with the retreating cliffs. As
the Vermilion Cliffs continue to retreat, a broad area
develops at the base of the cliffs. Since the Kaibab Limestone
dips toward the north, this newly exposed area can erode to
lower elevations. Eventually a new arroyo begins to develop on
this lower surface. Thus, the process repeats with time.
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