Erosion is the process by which sediments are transported by wind, water, ice, or gravity. Often people mistake erosion for weathering, the process through which rocks are gradually chipped away by abrasion, water, and ice into sediments, a topic that is studied in greater detail in the Geology Box. On the other hand, erosion is the movement of these sediments from one place to another. Erosion is a very natural and essential natural process upon which many ecosystems depend including beaches, deltas and wetlands.
The central aspect of this activity is the observation of differences in sedimentation patterns based on the velocity of water movement. The central principle is as water moves faster, it can carry larger sediments and more of them. The smallest sediment particles (silt and clay) are picked up first, followed by sand then gravel. Thus, a rain-swollen river will carry a great deal of sediments of all sizes while a slow, meandering stream will carry very little and only of the smallest sizes. As the current velocity slows, sediments are deposited in reverse order, gravel is dropped first, then sand, then silt and clay.
In most watersheds, rivers begin in the high mountains with steep slopes, and thus, with fast running water. The smallest sediments are quickly borne away, leaving behind the gravel and larger rocks. As the slope of the landscape lessens in the foothills, the river slows and sandbars may accumulate along the curves and twists of the riverbed. Finally, as the slope becomes nearly flat, either upon reaching the valley floor or as it reaches a body of water such as a lake or bay, the current velocity is reduced to a crawl and even the smallest sediments are deposited, leading to silt-covered deltas and clay-like mud covering the bottom of lakes and bays.
All of these processes may be observed in the river models in which silt from the faster flowing river channels is picked up by the water and deposited when the water slows in the bay at the base of the model, eventually forming a delta. The sand and gravel in the upper regions of the model are gradually exposed. If your students do the silt only river models described in the GEMS guide “River Cutters” they will soon discover that the rivers formed in the mixed sediment rivers are slower to form, straighter and shallower, more similar to a young river, owing to the higher percentage of gravel and sand which are not as readily carried away by the water.
In my classes, I spend 1 class period introducing the activity, conducting the experiment, and allowing students time to make observations and draw individual conclusions. I spend the second class period in a group discussion, leading students towards a clear theory to explain their observations.
Students need hands-on experience with soil separation tests in order to see how the smallest particles of sediment stay suspended in agitated water longer than larger sediments (see the Soil Analysis lesson). Although this is used as a demonstration before beginning this lesson, students recognize the patterns and their implications best when they make the discovery themselves.
In addition, it is strongly recommended that students complete a minimum of the first 3, and preferably, the first 5 sessions in the GEMS guide so that students have experimented with time (how long a river has been running) and with slope before adding the variable of multiple sediments to the model.