Stream Anatomy — Stream Bank

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Stream or Fluvial Geomorphology:
The study of how the geological features and shape of the stream channel change over time.
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Flowing water is a powerful force and as it carves out a channel it moves materials downstream, and sometimes changes course unexpectedly. The entire stream is in equilibrium which means that if one part is altered, the stream will work to regain equilibrium by altering another part. Several complex factors shape the stream channel. The most important are hydraulic force (flowing water) and debris load (e.g., rocks, wood, ice carried in water). Consequently, the stream moves a great deal of material downstream. The lighter, smaller sediment moves downstream faster than larger, heavier material.

Gradual bank erosion is a natural process that benefits stream life. It creates habitat diversity by creating pools, undercut banks, back eddies, and sloughs. It exposes tree roots and provides new sources of gravel. These features give character to the stream and provide complex habitat for the various stages in the life cycles of fish and other stream organisms. (text adapted from Pacific Streamkeepers Association; 2007)

Stream bank shapes

Types of stream bank shapes

The shape of the stream bank may include:

  • Vertical or undercut bank- a bank that rises vertically or overhangs the stream. This type of bank generally provides good cover for macroinvertebrates and fish and is resistant to erosion. However, if seriously undercut, it might be vulnerable to collapse.
  • Steeply sloping- a bank that slopes at more than a 30 degree angle. This type of bank is very vulnerable to erosion.
  • Gradual sloping- a bank that has a slope of 30 degrees or less. This type of stream bank is highly resistant to erosion, but does not provide much streamside cover.

Artificial bank modifications include all artificial structural changes to the stream bank such as riprap (broken rock, cobbles, or boulders placed on earth surfaces such as the face of a dam or the bank of a stream, for protection against the action of the water) and bulkheads. Determine the approximate percentage of each bank (both the left and right) that is artificially covered by the placement of rocks, wood, or concrete. (March 2003-Text adapted from the USEPA Stream Habitat Walk)