• 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 - Are the changes 'natural' or 'man-made'?

Streams and their accompanying riparian areas are inherently dynamic. They are always changing in some way both on an annual basis and over longer periods of time.

Although sometimes difficult, it is important to try to distinguish between changes caused by natural conditions and those which result from "non-natural" (human-induced) activities.

Natural Condition =
Suppose, for example, a long period of drought has lowered the groundwater table, and the water in a channel has been affected so that normal flow is much lower and the roots of some plant species are no longer able to reach water. The result might be a change in the plant community in the riparian area, probably from a more productive to a less productive one (Fig. 1-9). This would be an example of a natural stress on the system, and there might be little we as managers could do.

Man-made Condition =
On the other hand, if an inefficiently constructed irrigation diversion resulted in less water flowing through the channel on an annual basis, this would be a situation in which management actions have disrupted the normal flow and resulted in a similar change in vegetation. We might decide to restore the normal flow or decide to do nothing, but that is a management decision, not a cause of nature.

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