A meandering stream has a single, winding channel that twists and curves as it flows. Meandering streams typically form on relatively broad, shallow slopes allowing the channel to move freely across the landscape. A meandering stream contains enough stream energy to aggrade (deposit sediment) and degrade (remove sediment) the channel. These processes lead to distinctive landforms. For example, undercut banks form from degradation and point bars form from aggradation. The processes of degradation and aggradation cause the meanders (curves in the river) to migrate and occasionally narrow, cutting off a curve from the rest of the river. The cutoff meander becomes an oxbow lake. Oxbow lakes that fill in with sediment create meander scars on the earth's surface.
A thalweg is a line connecting the deepest points in a channel, and assists in determining stream energy and sediment transport. Aggradation typically occurs opposite the thalweg, and degradation usually occurs along the thalweg. Consider the information on channel form, and answer the following questions. 1. Using A and B in Figure 14.2, draw a circle around the site of aggravation and a square around the site of degradation. Then label A and Bas an undercut bank or point bar.
Stream Velocity and Channel Geometry The processes of aggradation and degradation are directly related to the stream veloc-ity and mean particle size in the channel (Figure 14.3). The velocity in a stream can be calculated as the slope (S) of a channel multiplied by the hydraulic radius (R) of the channel, divided by an empirical coefficient of surface roughness (n). velocity — (S)(R)
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Figtere 14.3. The Hillbtlii71/ carve displays the relationship hereto velocity end mean particle size.
The slope is the ratio of rise to run (Figure 14.4), and describes the gradient of the channel as it moves along sloping or mountainous landscapes. To calculate the rise, determine the difference in elevation between two sites. To calculate the run, measure the horizontal distance between the two sites, using the same units as used for elevation.