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Middle Fork of the South Platte River
near Hartsel, Colorado

Middle Fork of the South Platte River

Here is a single thread channel.  The entrenchment ratio is greater than 3. The measured width to depth ratio is 28. Sinuosity from air photo measurements is 1.7.

Single-thread Channels

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.

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