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Channel Processes: Stream Type Classification & Channel Stability

The Rosgen classification system does not assess stability, but rather describes various river types and quantifies their morphological parameters. When the values of these quantitative morphological variables depart from value ranges typical of a stable state based on dimension, pattern, profile and materials, the channel may exceed a "stability threshold" resulting in degradation, aggradation, accelerated lateral extension, avulsion and other instability consequences. The corresponding instability or dis-equilibrium often results in a morphological shift (evolution) to a new stream type.

The sensitivity of stream types to imposed changes such as increase in flow, direct disturbance and riparian vegetation influence varies by stream type. These relations by stream type are shown in Table 3 (PDF) (1 pp, 66K) and reflect differences in potential sensitivity, recovery, sediment supply, stream bank erosion and vegetative controlling influence (Rosgen 1994, 1996). Contrary to some misinterpretations, this general relation does not imply that D, G and F stream types cannot be stable, as they often represent the stable (but geologically active) form in many valley types. A departure analysis must be conducted to understand the stable reference reach form within a valley type.

Applications of this table can be demonstrated in a recent Minnesota Department of Natural Resources publication (Devore, 1998), wherein a landowner claimed that streambanks and riparian vegetation improved by grazing compared to exclusion of livestock. The grazed reach that exhibited great improvement (Figure 24) is an E4 stream type, whereas the reach that was excluded from grazing (Figure 25) was an F4 stream type.

Figure 24

Figure 24. E4 Stream type (from Devore 1998)

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Figure 25

Figure 25. F4 Stream type (from Devore 1998).

If Table 3 is used to evaluate interpretations of the two different stream types, the following summary in Table 4 depicts the major differences between these two gravel-bed streams (Rosgen 1994, 1996).

Table 4. Comparison of management interpretations between E4 and F4 stream types (Rosgen 1994, 1996).

Stream Type Sensitivity to Disturbance Recovery Potential Sediment Supply Streambank Erosion Potential Vegetation Controlling Influence
E4 Very High Good Moderate High Very High
F4 Extreme Poor Very High Very High Moderate

Exclusion of livestock on the F4 stream type would not result in a rapid vegetative response due to the poor recovery potential and reduced vegetative controlling influence. If the F4 stream type evolved (through active lateral erosion of the high banks) eventually to an E4 (the stable reference reach type) inside of the previous F4 type, then recovery potential and vegetative influence would change -- but then, so would the stream type. Without the benefit of the interpretations of stream classification, the conclusions that were drawn of the grazed reach being far more stable than the livestock excluded area, may come to the right conclusion, but for the wrong reason. Although good grazing practices improved the E4 stream type (good recovery potential), the poor recovery potential on the F4 is not related to the vegetation/cattle grazing program. The author has observed F4 stream types in north-central Nevada that excluded cattle for a ten year period without any visible difference on the grazed and un-grazed F4 stream type. When a stream type eventually evolves from an F4 to E4 stream type, however, the sediment supply decreases due to reduced bed and bank erosion.

Bedload sediment rating curve data, representing 55 individual Colorado rivers, were plotted indicating that bedload transport rates varied over six orders of magnitude for the same discharge (Figure 26) (Rosgen 1996). When the same data are stratified by stream type, much of the variability in the bedload transport relation can be explained by stream type (Figure 27) (Rosgen 1996, 2001b). Stream classification and channel stability ratings (Pfankuch, 1975) are coupled to further stratify differences in measured sediment rating curves, and to establish reference dimensionless bedload and suspended sediment rating curve data (Troendle et al, 2001). These methods are recommended for application in WARSSS.

Figure 26

Figure 26. Measured bedload sediment for 55 various Colorado Rivers (from Williams and Rosgen 1989)

Figure 27

Figure 27. Bedload sediment rating curves stratified by stream type; from the same data set as used for Figure II-25 (Rosgen 1996)

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