Physical Geology lecture - Fluvial and lacustrine systems
Geomorphology: "Geo" for earth, and "morph" for form, geomorphology is the branch of geology that looks at the earth's surface, and the various landforms that exist and the processes and history that led to their creation. Here, the a uniformitatarian approach is useful, as we can observe and measure many of these processes in real time (patience is quite necessary). This material on rivers and lakes is the first of a suite of lectures focused on geomorphology.
This is a view of the upstream portion of the Grand Canyon with the Colorado River visible down below. It is a simple but powerful example of the importance of rivers as a geologic agent. The Colorado river was and is the major agent by which the eroded material was transported somewhere else, leaving the canyon behind. Rivers not only consist of flowing water, but are rivers of sediment.
What are important factors you are aware of that determine how a river system behaves? What do we mean by behavior? When a river floods that might be considered bad behavior, and when it supplies us with drinking water that might be considered good behavior. But the behavior we are considering here is more general. How much water and sediment does it carry? How fast does the river carve a river valley or a canyon? What shape does the valley or canyon take? Why and how quickly do rivers change paths and meander? We could rephrase the question - what determines river system dynamics?
Fluvial geomorphology and dynamics
Rivers can occupy valleys and canyons of all sorts of cross section forms (V shaped, canyons with vertical walls, stepped walls). What controls these shapes?
Knickpoints (waterfalls and rapids) migrate upstream by a process known as headward retreat. The key aspect here is that erosive rivers not only cut down, they cut back into the landscape.
Image of one of the most famous knickpoints - Niagara Falls. Note the layered rock underlying the falls.
Image of explanatory diagram at Niagara Falls showing the various layers the falls are cutting into, and the process by which the falls migrates upstream. Shale is a much more erodable sedimentary rock than dolostone, and so, with time the shale gets removed creating an overhang, which eventually falls and then is ground up by the falling water. As this is repeated over and over again the falls retreat. It has retreated some 11.4 kilometers in the last 12,300 years, but because of the diversion tunnels used to produce electricity, the rate has slowed in recent times.
Meandering and the need to bend (water doesn't flow straight):
View of Matanuska River valley in Alaska. The modern flood plain visible below is a classic example of a braided (multichannel) system. The Matanuska River is upstream, and one can see the rich sediment source in the mountains in the distance. Note that we are standing on sands and gravels that underly a flat surface in the valley - these are old river deposits.
Elkhorn river near Scribner, Nebraska
Upstream view of Elkhorn River downstream of Winslow, with point bar sands to the right, and vegetated cutbank to left, and multiple channels within the main channel. Note the large trees. These influence flow and can often be found oriented with the water flow direction on the point bars and at the top of sand bars.
Large cutbank on outer bend of Elkhorn River near Nickerson. The material being cut into is old glacial and river deposits.
View of river terraces in the Madison River valley in Montana (famed for its fly fishing).
Another well developed flat topped terrace near Ennis Montana (on the Madison River). Note the mountains in the backdrop, which are created by a fault at their base and uplift on the mountain side. The change in cut and fill dynamics that produced the terraces could be influenced by the history of faulting and uplift.
Rivers are flowing water and sediment. How do rivers erode, transport and deposit sediments? There are three sediment transport mechanisms - a) bed load, b) suspended load, and c) dissolved load.
Drainage networks, a larger perspective
Various map patterns exist of the branching river systems: dendritic vs. trellis patterns are two examples. They are created by the process that controls the pattern of headward retreat.
Some examples of Nebraska drainages.
Lacustrine systems (lakes)
Lakes are efficient sediment traps, and fill up fairly quickly, on a geologic time scale, with sediment. So the question becomes - how do they form geologically; i.e. what are natural damming mechanisms or how are depressions created?
Dry Falls in central Washington state is one of the more spectacular geologic sites in the world, but one has to know the back story in order to understand why. The layers of dark rock here are basalt flows. Basalt is a fairly hard rock - not easy to erode. The cliffs you see across the entirety of the image here were, for short periods, the largest water falls in the world as the drained waters of glacial lakes in eastern Washington and Idaho swept through the area. During these breakout floods water would have been cascading over the width of the entire set of cliffs you see here. The lakes were the plunge pools, formed by the erosive power of the water at the base of the falls, which retreated.
The size of the material being transported gives some idea of the size of the flood. This image is down in the dry valley (coulee) downstream of Dry Falls. Behind the suburban, the large blocks of basalt were clasts over ten tons in weight that were being transported by the flood waters.
This map from the visitors center at Dry Falls shows the margin of the Ice Age glacier that dammed local topography, and the path of the flood waters that swept across the area leaving one of the most distinctive landscapes on earth, the channeled scablands.
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