My babbling brook flows through a narrow valley floor
that is about 150 yards wide, between valley walls that are very steep
and 100 feet high. The upper part of these steep walls are a bluff of
resistant Roubidoux sandstone. Upstream from my house site the brook
flows quietly on a bed of another, lower layer of Roubidoux sandstone,
a layer that is about eighteen inches thick. Along the edge of my yard
the brook becomes noisy as it tumbles through a mini-rapids where this
sandstone layer has been eroded away. Downstream the brook flows at
a lower elevation on a bed of gravel and dolomite bedrock.
Just above this mini-rapids the brook, which flows along
the west side of the valley, makes an abrupt turn, crosses the valley,
and flows along the east side. It maintains this position for only a
couple hundred yards then abruptly switches back to the west side. The
result is a sharp meander in a rather small stream valley.
I have tried to paint a descriptive picture to set
the stage for a series of questions. First, how did such a small stream
come to occupy such a large valley? What is the history of this little
stream? And what is the rate of change? How much has changed in my twenty
years of observing?
I have observed measurable change in my informal
study in only two places along the stream channel. The rest of the channel
is as far as I can tell essentially identical to what it was twenty
years ago. Most likely, if I had access to detailed photos of the area
over this time span I would see more changes; memory is weak.
The first change I have noted is that the sandstone
layer that forms the mini-rapids, the babbling part of my brook, has
retreated upstream ten to fifteen feet: a significant removal of resistant
rock. The second change is along the lower (downstream) part of the
meander. Here the stream has moved laterally into the valley wall about
five feet, and just downstream it has moved laterally into valley fill
sediments about ten feet, removing a couple of small hickory trees in
the process. But, when did this occur - did it happen at a millimeter
per day or suddenly?
This babbling brook, while it is babbling in normal
flow does NO eroding. The water is crystal clear except when a crawdad
kicks up some mud. The erosion and change occur during the three or
four days a year when heavy rains fall in the drainage basin (the 300
- 400 acres which drains into this valley) causing my babbling brook
to become a noisy full-fledged stream, bank-full with muddy, churning
water. But, the real change occurs perhaps once a decade, when a super-heavy
storm in the drainage basin turns my babbling brook into a roaring river.
A roaring river that fills the entire valley floor with sufficient force
along the main channel to uproot trees and pluck 200-pound sandstone
slabs loose from the stream bed and roll them smashing downstream. The
conclusion is that virtually all stream erosion occurs during those
super floods that happen on a time scale of once or twice per decade.
This applies to the major streams as well. Normal stream flow does very
little to no erosion.
At the place where the stream begins the meander
across its valley there is a large (6 ft. x 10 ft. x 12 ft.) boulder
of sandstone right where the stream bed would be if it didn't turn.
This large boulder is identical to the sandstone of the bluffs at the
top of the valley wall. The obvious conclusion is that a fragment of
the bluff rock broke loose and tumbled or slid down the valley wall
coming to rest in the stream channel. This formed a dam which diverted
the stream flow to create a new channel across the valley floor, and
a meander was born.
The "flat" bottom or floodplain in my valley would classify
it as a "mature" stream valley. Upstream about a quarter of a mile several
smaller tributary stream valleys merge to form one. These tributary
valleys are classified as young. They lack flat bottoms and floodplain
deposits. These young valleys have a "V" shaped cross-section and the
stream bed fills the narrow valley floor. These young valleys have a
steeper gradient and the main erosion is downward to make a deeper valley.
A mature valley has a shallower gradient with deposition of sediments
in the valley to form a flat floodplain. The main erosion in a mature
valley is lateral, back and forth across and through the floodplain
deposits.
But the questions remain, how old is this valley and how
long does it take to erode such a valley? I don't have a complete answer
to these questions. The minor changes I have observed along with the
accounts of the early explorers of this region who describe the valleys
much as they are today (except for vegetation) suggest that the valleys
are very old and the erosion rate very slow. Accounts of plant communities
in some deep isolated Ozark valleys which resemble plant associations
similar to ice age conditions hint that these valleys were here over
ten thousand years ago during the ice ages. This is no surprise to geologists,
because the Ozark region has been subject to erosion for millions of
years. The question is, when did the present cycle of erosion begin?
At locations in eastern Missouri a layer of gravel caps the higher elevations.
This gravel layer is thought to be Pliocene in age (5.3 - 1.8 million
years ago). The suggestion is that these Pliocene gravels were deposited
on a relatively flat regional erosional surface. It is not known if
this pre-Pliocene erosional surface existed in the Bryant area. If it
does, then perhaps the current erosion cycle dates from then. The answer
is likely very many thousands or millions of years ago, but we simply
don't know.
