The adverse consequences of accelerated sediment supply, accelerated bank erosion rates, degradation, aggradation from channel disturbance, streamflow changes, sediment budget changes and many other causes can lead to channel change. These changes result in stability shifts and adjustments leading to stream channel morphological changes, resulting in stream type changes as shown in Figure 40 (Rosgen 1994, 1996).
Figure 40. Adjustments of channel cross-section and plan-view patterns, as stream types change or shift through a series of successional cycles.
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The stream type succession sequences in Figures 40 to 42 are similar to the stages of channel evolution as reported by Schumm et al. (1984) and Simon and Hupp (1986), but these figures present more change scenarios than the original channel evolution concept. The relation between successional stages and stream classification are shown in Figure 40 (Rosgen 1999). The nine scenarios in Figure 42, in involving successional sequences of stream type change, indicate a larger range of possible morphological shifts and their tendency toward stable endpoints than previously published (Rosgen 1999, 2001b).
Figure 41. Comparison of channel evolution model stages of Simon and Hupp (1986) with one morphological sequence of Rosgen stream types (from Rosgen 1999).
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Each stage of individual sequences, as shown in Figure 41 and 42, is associated with unique quantitative relations of morphological, hydrological, sedimentological and biological relations. The adverse adjustments due to an assortment of morphological sequential shifts in equilibrium can create accelerated sediment yields, loss of land, lowering of the water table, decreased land productivity, loss of aquatic habitat and diminished recreational and visual values. The WARSSS procedure identifies the existing stream type, associated channel adjustment scenario and the current stage of a stream reach in the successional sequence. In assessing stream reaches where excess sediment is associated with a dis-equilibrium condition of a particular stage, the succession relations assist in determining the following information:
- The appropriate morphological scenario
- Within a scenario, the current state or stage of the successional stage of the existing stream type
- The various stages generally associated with a succession endpoint
- The series of natural changes that occur prior to reaching stability (quasi-equilibrium)
- The stable end point of the channel type
Once the appropriate sequence is identified within a range of scenarios (Figure 42), the current stage of the existing stream type within that sequence is identified.
Figure 42. Various channel evolution scenarios involving stream type classification.
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This relation indicates the changes in stream types that can occur as the stream seeks a morphological form with greater stability or quasi-equilibrium. For each stage there are large arrays of morphological variables that describe the current dimension, pattern and profile of the stream type. An example of a stable C4 reference reach with excellent woody riparian vegetation and negligible channel erosion is shown in Figure 43. An unstable C4 reach, with poor riparian woody vegetation and accelerated streambank erosion, appears in Figure 44. These examples represent the same stream type, but the latter indicates potential channel change from a C4 stream type to a D4 stream type as depicted in Scenario #2 in Figure 42. The bank erosion can often continue with channel enlargement and aggradation with the resultant stage of a D4 or braided stream type (Figure 45).
Figure 43. A stable C4 stream type associated with excellent riparian vegetation.
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Figure 44. Unstable C4, showing higher "w/d ratio" due to accelerated streambank erosion. Note grass/forb vegetation.
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Figure 45. An unstable D4 stream type exhibiting multiple thread channels, extremely high w/d ratio (7200) and accelerated bank erosion.
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The sediment consequences associated with dis-equilibrium are evident in the previous photographs as the channel types change from a stable C4 to an unstable C4, and then to a D4 stream type. The beneficial uses of water for fish habitat, recreation, and aesthetics are all adversely impacted with these changes. It may take a long time, but eventually the stream, with proper riparian vegetation, will restore itself to a C4 stream type. Wolf Creek, described previously, changed from a C4 stream type (Figure 30) to a D4 stream type following willow eradication (Figure 31). The channel succession was typical of scenario #2 from Figure 42. Another series of channel changes due to instability is related to scenario # 1 (Figure 42), involving degradation and lateral extension processes. This particular sequence is shown in Figures 46 to 49.
Figure 46. Channel succession stage from E to an unstable C, note increase in w/d ratio.
Figure 47. After the unstable C degrades to a G, the stage shifts from G (low w/d) to F (high w/d).
Figure 48. Channel succession stage shift from unstable F to more stable C. Bed of the former F is the new floodplain for the C stream type.
Figure 49. Succession stage showing C to E as vegetation reduces w/d ratio
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The sediment consequence of instability is quite evident in these photographs due to sediment sources from both the bed and banks of the G and F stream types. It is important to recognize the central tendency of rivers to seek stability as shown in the eventual recovery to the diminished sediment supply associated with the more stable C and E stream types. Sediment consequence data will be presented for Wolf Creek and the Weminuche River as case examples in the application of WARSSS.
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