Rivers
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[edit] Introduction
In this article we shall discuss the action of rivers on the landscape, and show how the characteristic sediments deposited by them can allow us to identify ancient river courses in the geological record.
The reader should note that there is no qualitative difference between a stream and a river; a stream is simply a small river, or, to put it another way, a river is a big stream. Rather than write "rivers or streams" over and over again, we shall write about rivers, and the reader may assume that what we have to say applies on a smaller scale to streams.
This article will employ some technical terms introduced in our article on Mechanical Weathering and Erosion and our article on Sedimentary Rocks.
[edit] Braided and meandering rivers
Rivers, unless artificially banked, rarely flow in completely straight lines. At low gradients, two characteristic forms they can take are braided and meandering rivers.
Braided rivers, as the name suggests, consist of a number of channels which separate and rejoin around bars of sediment. They are formed when a river with a lot of sediment repeatedly deposits the sediment and erodes it; so the braids and bars are not permanent features, but shift around over time.
The video below, produced by Dr. Paul Heller of the University of Wyoming, shows the process of formation of braided channels in miniature: it was produced in a flume 3 meters wide and 6 meters long fed with sediment-laden water. The video took 45 minutes to make, but has been speeded up by a factor of 30:1.
If the tendency to increase the curvature goes far enough, the meander approaches a loop doubling back on itself, as can be seen in the photograph above right of the Nowitna River, Alaska. If this process goes any further, the meander will touch itself and the river will suddenly find itself with a new, straighter, shorter path, leaving the meander isolated as an oxbow lake; such lakes can be seen in the foreground and background of the photograph. Being isolated from the river, these lakes will often then dry out or fill up with sediment.
[edit] Superposed and antecedent rivers
In many places in the world, we can find rivers which have cut channels through ranges of hills or mountains. Students of creationist nonsense may recall that this is the basis for a fairly characteristic piece of creationist illogic, as follows:
- I can't see how these channels were formed.
- Therefore, I know exactly how these channels were formed: they were formed by Noah's Flood.
Typically, such creationists do not go on to explain how water with a miraculous cause could achieve what they maintain that water with a non-miraculous cause cannot. The actual explanation does not, of course, involve miracles, and should be evident to anyone willing to apply a few seconds' thought to the situation: the rivers were there before the mountains.
In the case of antecedent rivers, tectonic uplift slowly raises hills or mountains across the path of the river. So long as the river can erode away the uplifted rock and soil as fast as the rate of uplift, it will maintain its course. As rates of uplift are small compared to the erosional powers of rivers, this should present no problem to a river of reasonable velocity.
In the case of superposed rivers (also known as superimposed rivers) a river flows over a plain, subject to weathering and erosion, beneath which are humps (antisynclines) of rock more resistant to erosion than the overlying rock. As erosional processes reveal the resistant antisynclines, the river cuts its way through them, resulting in a river that cuts through hills consisting of the exposed resistant antisynclines. An aerial view of the Susquehanna River, Pennsylvania, can be seen here; the exposed ridges of resistant rock are clearly visible.
[edit] Sedimentary structures
The sedimentary structures formed by a river at a particular point will depend on its velocity, its depth, and the sediment type. Geologists can discover the relationship between these factors both by observing actual rivers, and by laboratory experiments using flumes.
For example, consider the effect that a river's velocity has on a bed consisting of average-sized sandgrains. At low velocities, the creep of sand along the bed will, if anything, tend to smooth out the bed.
At higher velocities, sand ripples begin to form: small ridges of sand with the ridge at right-angles to the current. These ripples have a characteristic profile with a shallow slope on the upstream side and a steeper slope on the downstream side. Saltation bounces particles of sand up the shallow upstream side and over the peak of the ripple, eroding the upstream side and depositing sand on the downstream side. This has the effect that the ripples march downstream; it also produces cross-bedding.
At a higher velocity still, dunes (essentially, big ripples) will form; as with ripples, they have a shallow slope on the upstream side and a steeper slope on the downstream side. Dunes formed in this way have ripples on their shallow upstream side; these are known, logically enough, as rippled dunes. As with ripples, transport by saltation moves the dunes and ripples downstream (with the ripples moving rather faster than the dunes) and produces cross-bedding.
At greater velocities still, especially when the sand is fine, the increased current will flatten out the ripples, resulting in a flat, stratified surface known as an upper plane bed.
At still higher velocities, antidunes forum. These have a rounded undulating cross-section. With antidunes, sand is eroded from the downstream side of the antidune, and deposited on the upstream side of the next antidune. This has the effect that although the sand is moving downstream, the antidunes, being eroded on their downstream sides and built up on their upstream sides, move upstream: this is why they are called antidunes. These may show some slight cross-bedding, which, if it occurs, will slope up in the downstream direction; again, the opposite direction to that seen in ripples and dunes.
At greater velocities still, the current is strong enough to carry the sand in suspension, moving it downstream, leaving only gravel, cobbles, or just plain bedrock, depending on what other sediments, if any, are present on the river bed.
As we have indicated, the type of sediments involved affect these processes: in fine sediments, which "flow" more easily, dunes will not be formed; in coarser sediments, especially in shallow water, the formation of an upper plane bed is less likely, and the sequence as velocity increases will skip straight from dunes to antidunes.
Some photographs of these structures in modern sediments and in ancient sedimentary rocks can be found here.
[edit] Vanished rivers: how do we know?
Geologists can reconstruct the courses of long-vanished rivers. The method by which they identify them should be obvious and familiar to anyone who has followed our series of articles on geology.
If we take away a river, we are left with its sediments, which will eventually lithify. This will leave us with a set of rocks which look just like the lithified sediments of a river:
- They will be arranged in the long thin form of a river (what is sometimes called a "shoestring topography".
- They will have the sorts of sediments and sedimentary structures that we associate with a river.
- Where these sediments indicate which way way upstream, and which way was downstream, as such sedimentary structures often do, they will point along the direction of the former river.
- The types of sediment will be consistent with the hypothesis of a river. For example, we will not find limestone, or chert, or gypsum, or halite, because these would require totally different depositional environments.
- Such fossils as we find will be of freshwater plants and animals, or of land plants and animals, but not marine forms. Similarly on the banks of the river we expect such fossils as are present to be of land animals or their footprints, and of land plants.
In short, we see exactly what we should expect to see as the remains of a former river. As usual in such cases, we are left either with the reasonable deduction that these features are, in fact, the results of the action of a river now vanished; or with wild, bizarre hypotheses such as that a malevolent god is playing pranks on geologists.
