There are three basic types of channels, straight, meandering and braided. Describing a channel by one of the aforementioned terms does not mean that the entire channel is straight or otherwise. It simply means that some portion of the channel can be described in such a way. In fact, portions of a stream may be straight, some meandering and others braided.
Figure 18.25 The sinuosity ratio
Describing a channel as a straight channel seems pretty obvious, though rarely is a channel perfectly straight in nature. A meandering channel is one that takes twists and turns over its length. Geoscientists use the sinuosity ratio to determine whether a channel is straight or meandering. The sinuosity ratio is the distance between two points on the stream measured along the channel divided by the straight line distance between the two points. If the sinuosity ratio is 1.5 or greater the channel is considered to be a meandering one.
Figure 18.26 Braided river at junction of Gakona and Copper River, Alaska
A braided channel is created when a stream channel is divided into several smaller ones by the accumulation of in-channel deposits. This occurs when the load of flat stream channel is too great for the velocity or discharge. Or, seasonal fluctuations in discharge expose in-channel deposits. Sand or gravel bars accumulate subdividing the flow of water into many smaller channels. Braided streams are common in glaciated areas where melt water streams choked with sediment is discharged at the snout of the glacier.
Pools and Riffles
We often find a regular sequence of shallow riffles and deeper pools in stream channels, the cause of which is still not well understood. The spacing of the riffle-pool sequence is related to the width of the stream. Riffle-pool sequences usually are 5-7 times the width of the channel. Laboratory experiments with artificial channels in noncohesive sand or silt show that riffle and pool sequences in straight channels tend to evolve into meanders. When this happens, a pool becomes a site for a laterally migrating meander. The stream thalweg meanders back and forth between pools, moving toward the outer bank of each successive curve. Erosion is therefore concentrated at the outside banks where the flow is deepest and stream velocity accelerates around meander.
Assess your basic understanding of the preceding material by "Looking Back at the Stream System, Channel Geometry, Types and Flow" or continue reading.
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