Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 131

continues and the river valley widens. Because degradation flattens the channel gradient, the available
stream power for a given discharge reduces with time. Concurrently, bank heights increase and bank
angles steepen, which induces bank failures and successive landslides. Especially during earthquakes,
many rockfalls, avalanches, and landslides occur, causing the valley to broaden laterally. The valley takes
a broad “V” form in this stage. Because the valley becomes wider and the bank slopes gentler, humans
can reclaim the hill slopes, build houses, plant crops, and construct roads and highways; (c) As
degradation migrates further upstream, aggradation becomes the dominant trend in previously degraded
sites, because the flatter gradient cannot transport the increased sediment loads emanating from upstream.
The river bed becomes flat and the valley takes a “U” form. The valley becomes safer for humans to
reclaim and live in than in Stage 2; (d) Attainment of a new dynamic equilibrium takes place through ķ
bank widening and the consequent flattening of bank slopes, and knickpoint establishment through
landslide occurrence, quake lake formation, or in some cases dam construction by humans, ĸ sediment
deposition and bed structure development, and the establishment and proliferation of riparian vegetation
that adds roughness elements, enhances bank accretion, and reduces the stream power for a given
discharge, and Ĺ flow energy consumption at knickpoints and bed-gradient reduction in the reaches
upstream of the knickpoints.
Figure 3.11(a) shows an example of a tributary of the Minjiang River in stage 1. In this stage, deeply
incising rivers often create a landform called an inner gorge at the interface between hill slopes and river
channels. An inner gorge is characterized by a convex break in the hill-slope gradient, and lined by
hill-slope toes significantly steeper than those of upper valley flanks. Thus, the hill slope is divided into
the upper slope and the inner gorge wall (Korup., 2006). Figure 3.11(b) shows an example of the
Dajinchuan River in Sichuan in stage 2. Figure 3.11(c) shows an example of the Kuaihe River in the
upper Yangtze River basin in stage 3. Figure 3.11(d) shows an example of an upstream reach of the Dahu
River in Sichuan in stage 4. The valley is wide and flat. Several knickpoints consume energy and the
gradient between the knickpoints is low. The valley takes a broad “U” form, which is suitable for humans
to live and facilitates the best stream ecology.
Fig. 3.10 Four stages of the morphological process of incised rivers. ˄ a ˅ Rapid degradation stage showing a narrow
“V” form valley; (b) degradation and widening stage showing a broad “V” form valley; (c) widening and resiltation
stage, showing a “U” form valley; and (d) equilibrium stage showing a broad “U” form valley
3.1.3.4 Riparian Vegetation
In many incised stream systems, riparian vegetation plays an important role in flow and near-bank
hydraulics, bank stability, and stream habitat. In humid environments, the sequence of processes and