• Introduction
• Watershed importance
• Stream classification
  • Method 1
  • Method 2
  • Method 3
  • Method 4
• Stream mechanics
  • Energy
  • Natural vs Man-made
  • Stability
• Laterial Stability
• Vertical Stability
• Stream Changes
• Summary
• Interactive Exercises
• Do it yourself

Streams and Watersheds - Lateral Instability

There are two types of instability caused by streams which are trying to reestablish their equilibrium: lateral and vertical.

Lateral instability =
Lateral instability refers to a stream which is trying to slow itself down by cutting horizontally across the floodplain. This action is reflected in the presence of unvegetated, raw streambanks on the outside of curves or, in extreme cases, along straight stretches of the channel. Not all streams with cut banks are unstable. Low gradient streams in certain soils, for instance, naturally meander back and forth across their floodplains.

One way to determine if lateral cutting is inherent in a dynamically stable stream or represents an unstable condition is to monitor the width of the stream channel. On a naturally meandering channel, new point bars will be extending into the channel on the inside of curves at about the same rate the banks on the outside of curves are eroding away. Thus, the width of the channel remains constant over time (Fig. 1-12). In contrast, an unstable laterally cutting channel becomes wider over time.

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