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Earth Geology Erosion Contemplating a Babbling Brook

Contemplating a Babbling Brook 

In this essay, Verl Smith, Ava High School teacher and writer of the Geology section, describes how stream erosion happens by taking a good look at the brook beside which he lives.
 
I live by a stream, actually no more than a babbling brook, which I have been observing for over twenty years. This brook during normal flow has a depth measured in inches and a stream channel in most places two to three feet wide. It contains occasional small pools which are one to two feet deep. This small spring fed stream flows continuously in its deep valley toward the northwest. Approximately 500 yards beyond my home the stream submerges; it ceases to be a permanent surface stream and instead continues to flow through and beneath a bed of gravel. A couple hundred yards further my stream merges or joins another larger intermittent stream which flows to the northeast toward a rendezvous with White's Creek. Flow in this larger stream is, except during wet weather, entirely below the surface through its gravel bed and the gravel of former channels in the valley. 

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. 

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