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Functions
& Conditions -
Evaluating the Physical Features
The primary
physical features to consider in determining the health of a riparian
area are: (1) evidence of channel downcutting; (2) evidence of channel
widening; (3) amount, location, and causes of bare ground; (4) amount
of fine materials on the bottom of the channel; and (5) amount of
rock (larger than gravel size) which makes up the streambanks.
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Figure
2-10
Downcutting changes the hydrology and vegetation of
an area by lowering the water table and creating a narrower,
less stable channel. |
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Downcutting
Downcutting
of a channel, often referred to as incisement, is an indication
a riparian system may not be functioning properly. Downcutting occurs
when something causes the stream to increase its velocity and erode
away the channel bottom. An incised channel (as shown in Figure
2-10) is one in which the average flood--of the size and intensity
which occurs roughly every two years--is unable to "access
its floodplain" by overflowing its streambanks. As the channel
cuts downward, the groundwater table is lowered. Consequently, water-loving
plants isolated on the old floodplain and streambanks may no longer
get the moisture they need.
Over time, the
vegetation communities will change from riparian to non-riparian
species, which generally are not so productive. Often, they do not
perform adequately the functions of riparian vegetation discussed
above. In addition, because the water can no longer reach the floodplain,
the floodplain and streambanks no longer serve as a sponge to store
excess water during high runoff. As the water in the channel increases
its speed, it increases the potential for erosion of streambanks
and for sedimentation. As the stream downcuts, it creates head cuts.
As the water seeks a new, lower level, these will move up the main
channel as well as up side drainages. The result can be an unstable
watershed.
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Figure
2-11
This downcut channel has stabilized and is developing
a floodplain on the bottom of the old incisement. |
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A common situation
in the West is shown in Figure 2-11. Here past events created an
incised channel and changed the relationship of the stream to its
surrounding landscape. The water can no longer get near the top
of the old streambanks. As a result, vegetation on top of these
old streambanks has become an upland community. However, the stream
apparently has ceased downcutting, and a new riparian area is developing
in the bottom of the incised channel. The stream can reach this
new floodplain. The vegetation established along the new streambanks
and on the new floodplain is performing the functions of a riparian
community.
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