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Streams
and Watersheds - Stream Mechanics
Energy
Water
influences its surroundings through the energy it possesses. A basic
law of physics is that energy (force) equals mass (weight) times
acceleration (velocity). Water has weight. As it runs downhill,
it acquires velocity. The energy which develops results in "work,"
reflected by such actions as erosion of soil from streambanks or
stream bed and transportation of the sediment in the water column
or rolling along the stream bed.
The
biggest influence on the amount of energy a stream has, and thus
how much work it does, is speed. The speed, or velocity, of a stream's
water column depends on the slope (steepness) of the channel, the
depth of the channel (the deeper the water body, the greater its
mass), and the amount of resistance (friction) exerted on the flowing
water by bed materials, streambanks, and vegetation.
Friction slows the speed of water and thus reduces the amount of
energy it has and the work it can do. As water slows down or the
amount of flow is reduced, it loses energy and deposits sediments
and debris. The importance of resistance in reducing the velocity
and hence the work capacity of water in a stream highlights the
importance of streambank and floodplain vegetation (Fig. 1-8).
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Figure
1-8
Pioneering herbacious plants can affect channel shape
and riparian function by slowing water and trapping
sediments. |
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A
stream which flows along banks that have no vegetation and only
smooth surfaces such as small particles or bare ground will move
faster and exert more force on those banks, while one which must
work against vegetation or large rocks will move more slowly. In
addition to exerting less impact on the streambanks, the slower
stream will be less capable of carrying sediments. As sediments
settle out, they may create new streambanks and point bars.
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