Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 125

Table 3.3 uses the reference codes of Table 3.2 to categorize the various causes by their effect and by
the type of incision, respectively, which is either upstream or downstream progressing or both (Galay,
1983). The most common example of downstream progressing incision occurs downstream from large
dams. One can also group the causes into those that increase energy and those that decrease the resistance
of the surface or channel. For example, anything that causes steepening of the channel increases stream
power and a stream's ability to incise. All of the geologic, geomorphic, climatic, and hydrologic causes
with the exception of B8 and B9 fall into the category of increased energy. In contrast, animal causes
reduce resistance by removing vegetation and channelizing flow. The human causes combine both effects
with the effect of increasing energy by increasing or concentrating flow (F3, F4, F12, F13, F14, F15) by
increasing gradient (F5, F6, F7, F8, F10, F11), and/or by decreasing sediment loads (F1, F2, F10, F11). A
single cause can produce either a gully or an entrenched stream or both, and the effect of the incision can
either propagate upstream or downstream or both.

T able 3.3 Effects and types of incision
Formation of a new
channel (gully)

Deepening of existing channel
(entrenched channel)

Downstream
progressing

Upstream progressing

A1, A2, A3, A4 A1, A2, A3 A1, A2, A3, A4

B2, B4, B6, B7, B8, B9 B1, B2, B3, B5, B6, B8 B4, B8 B1, B2, B3, B4, B5, B6,

B7, B8, B9

C1, C2, C3 C2, C3 C1, C2, C3 C1, C2, C3

D1, D2, D3 D1, D2, D3 D1, D2, D3 D1, D2, D3

E1, E2 E1, E2

F3, F4, F8, F9, F12,
F14, F15, F16

F1, F2, F3, F4, F5, F6, F7, F8,
F9, F10, F11, F13, F14, F15

F1, F2, F3, F4, F10,
F11, F12, F14, F15

F3, F4, F5, F6, F7, F8,F9, F10,
F11, F12, F13,F14, F15

3.1.2.1 Geological Causes
Uplift, subsidence, and faulting all modify the slope of the valley floor and channel gradient. For
example, channel incision should occur on the downstream steeper part of uplift and at the upstream steeper
side of subsidence and where a stream crosses from the up-thrown to the down-dropped portions of a
fault. If the uplift is a dome, gullies can form a radiating pattern. Lateral tilt of a valley floor can cause
avulsion and the development of a new channel, and lateral fault displacement can also cause stream
incision and gullies.
The 921 earthquake in Taiwan (September 21, 1999) was caused by tectonic motion. The tectonic
motion generated a huge fault, and caused lateral tilt of river valleys. Figure 3.4 shows that the left side
of a river in Taiwan, has risen up and the right side of the river has settled down by 8 m due to the tectonic
motion. Erosion and incision of the channel bed was induced thereafter. The magnitude of the resulting
incision depends on the amount of deformation and the ability of a channel to adjust to the altered slope
by increasing its sinuosity (Schumm, 1985).
According to the British Geological Survey, the Indian Plate moves northward at a rate of 5 cm/year
(Chen and Gavin, 2008). Its collision with the Eurasian Plate has resulted in the uplift of the Himalaya
and the Qinghai-Tibetan plateau, and associated earthquake activity. The rate of horizontal movement is
about 5 cm/yr at Himalaya, 4 cm/yr in the source area of the Lancang and Yangtze Rivers, to about 2
cm/yr at the Qilian Mountain. As a result the Himalaya rises at a rate of 21 mm/yr while the Qilian
Mountains rise at a rate of 5 mm/yr. The Sichuan basin, located to the east of the plateau, is stationary.
This pronounced variability in the rates of horizontal movement has resulted in many active faults and
river bed incision of almost all rivers on the east margin of the plateau.