The measured water surface slope is
.018 feet per foot; and the D50 size of channel material is 38 millimeters, in the gravel
category. This reach classifies as a C4 stream; a very common "riffle-pool"
stream type in Region 6.
Now put this classification to
practical use. Notice the width to depth ratio of 28. A stable C4 stream, as determined
from reference reach data, has a w/d within the range of 12 to 18. The high width to depth
ratio of the pictured stream is an indication the stream has over widened as a consequence
of some change and exhibits a w/d ratio beyond the stable range for a C4 stream type. The
cause in this case is willow removal and heavy cattle use of the stream bank. The
correction, based on field experience with similar rivers, would include riparian corridor
fencing, better cattle management, and reestablishment of willows. With these corrections
in place, the stream is likely to narrow and deepen over time, as it adjusts to a stable
width to depth ratio. This outcome is likely because the stream is not drastically over
widened and therefore the stream's inherent ability to "heal itself" may be
sufficient.
In this classification exercise, the
reference reach "blueprint" provided the basis for comparison with an existing
channel. The reference reach is the "compared to what" benchmark.
The stable form width to depth ratio of 12 to 18 is the blueprint from which to assess the
28 width to depth ratio in the existing channel. This example illustrates that
accurate stream classification coupled with data from reference reaches is the basis for
subsequent assessment of stream condition.
