-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
Stream Erosion
Erosion is the process by which water, wind, ice, and gravity shape the Earth's landforms. Running water is one of the most significant erosional processes. Not only does the action of moving water itself erode soil and rock, sediment transported by running water abrades the surfaces that the water contacts. Specifically, rivers, streams, and waves transport various sizes of clay, silt, sand and gravel, and even cobbles and boulders at times. As these materials are swept along by the water, they grind against one another and any surface they contact. The result of such abrasion varies based the material being eroded. For example, a stream in soft sediments will erode more quickly than a stream through solid rock; the same is true for the erosion of a sandy beach versus that of a rocky coastline.
Rivers and Landscape Shaping
Rivers do most of the erosional work of landscape shaping. Rivers have been credited with 85 to 90 percent of the total sediment transported to the sea. (A grain of sand may require 1,000 years to be carried through a large river network to the sea.) This contrasts with about 7 percent transported by glaciers, 1 to 2 percent by groundwater and ocean waves, and less than 1 percent each by wind and volcanoes. Presumably, these volumes of sediment transport are proportional to the relative volumes of landscape eroded by the various agents. Therefore, any general consideration of landscape development must deal primarily with the work of rivers.
Most erosion by rivers is accomplished during the brief intervals of high discharge and flooding. At these times, rivers flow not only faster, deeper, and wider, but also much muddier. Because of the great increase in turbulence in a deep, fast-flowing stream, it can carry one hundred to one thousand times more sediment than it can at low-water stages. Thus, if a river at flood has one thousand times as much water, and that water is one thousand times muddier than at low flow, then one million times more sediment is being moved. Such numbers are typical of actual measured values.
Rivers erode their channels by:
* Grinding the rocky channel with abrasive particles already being carried;
* Plucking, or tearing out large blocks along preexisting fractures; and
* Dissolving rocks (such as limestone).
As with sediment transport, most riverbed erosion occurs during brief times of high discharge. By far, however, the most sediment in a river comes from the hill slopes on the valley sides rather than by direct river action. In that sense, rivers act more like passive gutters or storm drains that are forced to carry water and sediment that are delivered to them from their drainage basins rather than actively producing their own.
Duck Creek Examples:
Please click on the thumbnails of the photographs to see the full size images!
The major erosion in a mature valley is lateral as the stream shifts course back and forth across the valley and cuts into the valley walls to widen the valley itself. The shortest course for a stream is a straight line and water will follow that shortest course when possible. However, several factors work to create a meandering stream and deflect the flow away from a straight line. One factor is streambed clogging due to heavy sediment dumping during a flood. Sediment may pile up, causing the stream to seek a new channel across a different part of the floodplain. This usually results in minor shifts of streamflow, but can create significant changes. Another possible factor is landslides or the collapse of an oversteep valley wall or bluff, which creates a dam that may divert the stream flow to the other side of a valley and result in a permanent meander. Another cause is the shifting of bedrock along faults that cross the valley. This sudden change can create sharp and unusual meanders in a stream channel.
Once a stream channel begins to meander or curve there is a strong tendency to maintain and increase the curve. As a stream rounds a curve the greatest erosive force is located on the outside of the curve. The outside bank of the stream channel deflects the force of the running water along the curve and undergoes increased erosion. The inside of a curve is typically a zone of deposition as sediments are deposited from the slower current found there. In this way the stream channel tends to migrate laterally in the direction of the outside curve and the curve or meander grows. Occasionally erosion cuts through the neck of a sharp meander leaving an abandoned or cutoff meander valley.
You may notice that it is the outside of a curve that eats into the stream bank. The water flows fastest on the outside of the curve. On the inside curve, the slower water drops its sediment, and the land builds up on that bank. That's where you find gravel bars. Then sycamores sprout. In a few years what was stream bottom has become bottomland! An interesting erosion feature observed along stream channels is the undercut bank. In this instance, the stream erodes beneath a resistant rock layer along the outside of a curve and creates a cave.
In certain instances, pockets of rock erode quicker than the surrounding rock making small caves such as these:
Posting requirements:
1. Determine the speed at the top of the curve at N 41° 32.044 W 090° 29.188 in relation to the speed at the bottom. Take two biodegradable items such as leaves, small sticks, etc. Toss on at the top of the curve and time the distance the stick travels. Then time another stick at the bottom of the curve and report the two times divided by the distance travelled. Report both numbers and give a brief description of the experiment.
2. Determine the mode of erosion at N 41° 31.972 W 090° 28.938; Grinding, plucking, or dissolving. Explain why you think it was the method you chose.
3. Report these observations to IowaBeaver through the geocaching website.
Bibliography
Bloom, Arthur L. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1998.
Ritter, Dale F., R. Craig Kochel, and Jerry R. Miller. Process Geomorphology, 4th ed.New York: McGraw-Hill, 2002.
http://www.aegweb.org/i4a/pages/index.cfm?pageid=4083#examples
http://www.watersheds.org/earth/valley.htm
http://www.watersheds.org/earth/morevalley.htm
Additional Hints
(Decrypt)