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Active Tectonics: Studies in Geophysics
(pattern 4) and a typical braided-stream (pattern 5) complete the sequence. The relative stability of these channels in terms of their normal erosional activity and the shape and gradient of the channels, as related to relative sediment size, load, velocity of flow, and stream power, are also indicated on Figure 5.1. It has been possible to develop these patterns experimentally by varying the gradient, sediment load, stream power, and type of sediment load transported by the channel (Schumm and Khan, 1972).
The range of channels from straight through braided forms a continuum (Figure 5.1), but experimental work and field studies have indicated that the pattern changes between braided, meandering, and straight occur at river-pattern thresholds (Figure 5.2). The pattern change takes place at critical ranges of valley slope, stream power, and sediment load (Schumm and Khan, 1972).
Observed rivers can be placed within the five general categories. However, within the meandering stream group there is considerable range of sinuosity (1.25 to 3.0), which is the ratio of channel length to valley length. In addition, in the braided-stream category there are bar-braided and island-braided channels. Islands are vegetated bars. There are also multiple channel patterns termed anastomosing, anastomosed, or anabranch channels (Schumm, 1977, p. 155; Smith and Smith, 1980). In fact, it has been suggested that 14 channel patterns can be recognized (Figure 5.3).
Experimental studies and field observations confirm that a change of valley-floor slope will cause a change of channel pattern and dimensions. The change will differ, however, depending on (1) where the channel lies on a plot such as that of Figure 5.2, (2) the type of channel (Figures 5.1 and 5.3), and (3) the amount and rate of deformation.
FIGURE 5.2 Relation between valley (flume) slope and sinuosity (channel length/valley slope or valley slope/channel slope) during experiments at constant discharge. Sediment load, stream power, and velocity increase with slope, and a similar relation can be developed with these variables. From Schumm and Khan (1972).
FIGURE 5.3 The range of alluvial channel patterns for the three channel types shown in Figure 5.1. A, Bed-load channel patterns; B, mixed-load channel patterns; C, suspended-load channel patterns. From Schumm (1981).