Palo Duro Canyon is a classic example of a land-form that has been
created by the geologic work of _running water_. Undoubtedly the most
important single agent of erosion, running water probably does more to
wear away the land than all the other geologic agents combined. This is
not surprising considering the fact that the earth’s annual
precipitation (such as rain and snow) equals about four billion tons of
water. Although the amount of precipitation varies greatly from place to
place, the average annual precipitation on land is about 40 inches of
water. Of this, roughly 25 percent runs off from the land to form
streams.
When one drives through the park and fords the normally gently flowing
waters of the Prairie Dog Town Fork of the Red River he may well wonder
if this unimposing stream actually is the geologic agent that is
responsible for this deep gorge. But the visitor who happens to be
present during a severe rainstorm will soon be convinced, for during
heavy rains this gentle stream becomes a raging torrent. As the river
increases in size it also becomes a more effective land-shaping tool,
for the larger and swifter the stream, the more rock material it can
carry. Thus, when flowing at peak capacity, this branch of the Red River
becomes a moving ribbon of sandpaper whose load of sand, silt, and
gravel has cut and scoured the canyon walls and floor for hundreds of
thousands of years. How long has it taken the river to carve this
remarkable chasm? Although there is no way of knowing for sure, geologic
evidence indicates that the canyon has formed during the last one
million years—a relatively short time, geologically speaking.
The work of the river is made still more effective by water and sediment
which it receives from its tributaries; this added water substantially
increases the volume and velocity of the river. Although many of the
tributary streams are dry throughout much of the year, they carry large
quantities of water during heavy rains. Moreover, because most of these
streams flow over rock surfaces which are not protected by thick soil or
vegetation, their waters are quickly transported to the master stream.
Thus, the volume and velocity of the Prairie Dog Town Fork of the Red
River make it possible—especially during flood periods—for the river to
carry a large load of rock particles which effectively erodes the stream
channel. Where does this rock debris come from? Most of it is eroded
from the sides and bottom of the river’s channel.
The river carries its load in a number of ways. Material such as salt
and other soluble matter is transported in a dissolved state or in
_solution_. Still more, for example, silt and fine sand, is carried in
_suspension_. These sediments are suspended between the surface of the
water and the bottom of the stream channel. Those particles that will
not dissolve in water and are too heavy to be carried in suspension,
constitute the _bottom load_ of the stream. These larger sediments, such
as gravel, cobbles, and boulders, roll, bounce, or slide along the
stream bed.
As flash floods course through Palo Duro Canyon, the river uses its load
to erode further the rocks over which it passes. Each moving rock
fragment literally becomes a cutting tool for _abrasion_ as the loose
rock particles slowly wear away the banks and bed of the stream.
Eventually the abraded rock fragments become smooth and rounded and the
stream channel is gradually worn down to a lower level; it is also
widened.
The river also erodes by _hydraulic action_ as loose rock fragments are
lifted and moved by the force of the stream’s current. This process is
similar to the effect produced when soil is churned up and washed away
when water from a garden hose is sprayed on loose earth. The effects of
hydraulic action have played an important role in widening the canyon,
for recession of the cliffs away from the middle of the canyons has been
caused in part by undercutting. Thus, as the soft shale and gypsum beds
were removed by the stream, the overlying sandstone formations gradually
broke off and fell into the canyon. Once on the canyon floor, most of
the slabs and blocks of sandstone were eventually broken up and carried
away by the streams as sand and mud. Not all of the boulders have been
destroyed in this manner; in places (for example, the Rock Garden)
similar boulders are seen today (fig. 34).