Slight increases of valley slope will shift river patterns from left to right on Figures 5.2 and 5.3, as the river adjusts its gradient by pattern change. With greater changes of valley slope, incision may produce sufficient sediment to cause a change from one type of channel to another with a metamorphosis of a mixed-load channel to bed-load channel (Figure 5.3). In addition, significant reductions of slope or greatly increased sediment loads will produce aggradation and very likely a braided channel, as a result of sediment deposition.

Braided channels are the result of high bed-load transport on steep gradients or of deposition. Therefore, a braided channel can be in equilibrium or unstable. The anastomosing pattern (pattern 14 of Figure 5.3) is still an enigma. It may be a relatively steep gradient suspended-load channel analogous to the bed-load and mixed-load braided channels (Figure 5.3), or it may be the suspended-load equivalent of the unstable braided channel that forms where overbank flow produces multiple channels in a valley.

In an effort to determine the effects of active tectonics on alluvial channels experimental studies were performed by Ouchi (1983, 1985) and Jin (1984) in a large flume (8.5 m×2.4 m), the center section of which could be raised or lowered by hydraulic jacks. Figure 5.4 summarizes the results for braided and meandering channels during uplift and downwarping. Because the braided channel could not change its pattern, as a result of uplift, it degraded forming terraces. The sediment produced by the incision caused aggradation downstream, and the reduced gradient upstream also caused aggradation.

During downwarping the experimental braided channel degraded in the upper steepened reach, and it aggraded downstream. Adjustment was much slower during subsidence because during uplift channel incision is concentrated in a channel, whereas adjustment by aggradation requires deposition not only in the channel but over the valley floor. The aggradation in zones B and C reduced downstream sediment loads and induced degradation in zone D (Figure 5.4).

Adjustment of the meandering channel was as expected with increased overbank flooding upstream and an increase of sinuosity on the steeper reaches during uplift. Jin’s (1984) results also show clearly the meandering-channel response to uplift (Figure 5.5).

Note that in each case (Figure 5.4) the secondary response to the primary deformation causes tertiary effects in zones A and D both upstream and downstream of the zones of deformation (B and C). Figure 5.4 is presented to illustrate the complexity of the channel response to active tectonics. In each case if the experiment had continued without further deformation there

FIGURE 5.4 Effect of uplift and subsidence on braided and meandering experimental channels. Zones of active deformation (B, C) are separated by the axis of deformation. Zone A upstream and Zone D downstream from the zones of active deformation show tertiary effects of valley slope change. The position of the descriptive term indicates generally where the process or channel form is present. Modified from Ouchi (1983).

would be additional channel adjustment. For example, in the uplift experiments degradation, which was concentrated at the axis of uplift (Figure 5.4), would have extended upstream to at least the boundary between zones A and B.

Alluvial channels are sensitive indicators of change. However, they adjust to changes of hydrology and sediment load as well as to active tectonics. Therefore, it may be difficult to determine the cause of channel change because man’s activities and climatic variations both act to alter discharge and sediment load during historic time. Channel pattern change alone is not sufficient evidence for active tectonics, rather it is one bit of

FIGURE 5.5 Meandering channel pattern change, as a result of uplift in center of figure. A shows meandering channel developed after 400-h run time, B shows effect of uplift on this 3.5-m reach of experimental channel after an additional 100 h. Upstream reach shows evidence of overbank flooding, development of multiple channels, and aggradation. Downstream reach shows increase of meander amplitude, wavelength, and sinuosity as well as degradation and a cutoff. Flow is from left to right. Stippled pattern represents sand on floodplain. Vertical-line pattern represents silt and clay deposits on floodplain. From Jin (1984).

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement