Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 167

large landslides have occurred during earthquakes or rainstorms. A stable landslide dam stabilizes and
blocks a river valley, either initially or after some degree of catastrophic failure and dam-outburst flood
erosion. Step-pool system developed on landslide deposits consisting of a high percentage of large (>1 m)
boulders often lead to the formation of stable landslide dams and knickpoints.
A landslide dam creates a quake lake or barrier lake on the stream. Once upstream lake levels reach
the top of the landslide dam and water flows over the landslide deposits and erosion-resistant boulders
armor the channel bed as finer material is washed away. This process condenses the original landslide
material to a smaller mass composed of large boulders, stabilizing the landslide dam, and protecting the
top of the initial deposit from further erosion. These large boulders are not easily moved in even large
floods, and serve to roughen the bottom of the channel. There is often an evidence of advanced fluvial
sculpting of boulders, which attests to long periods of boulder stability (Ouimet et al., 2007). Upstream
of landslide dams, river gradients are low, and fine-grained lake sediments and alluvial gravels accumulate.
The abrupt change in slope associated with the transition from upstream, low-gradient fills to steep,
dramatic rapids through the landslide deposits creates significant knickpoints with a drop in elevation of
up to 100- to 300-m (Korup, 2006). The integrated effects of large landslides on river channels completely
prohibit rivers from eroding their bed and incising over the length of the landslide mass and associated
fill deposits.
The period of local non-incision continues for the entire duration of a landslide dam event, from the
emplacement of the dam to complete incision through landslide deposits and some portion of the
associated upstream fill. During this time, the long-term evolution of the river profile and landscape
evolution are affected in two main ways: ķ downstream reaches continue to incise, while landslide
reaches do not, and ĸ upstream reaches fill and the whole profile upstream becomes flatter.
The Gesudza He, a tributary of the Dadu River, is dominated by landslide dams, boulder rapids, and
impounded alluvial fills for 40 km upstream of its confluence with the Dadu River main stem (Fig. 3.48).
The most dramatic of these landslide dams occurs 15 km upstream from the confluence with the Dajin
Chuan, with a 150 m drop through the rapids caused by the landslide deposits. Field surveys show nicely
the characteristic drop in channel width and increase in channel slope across the landslide mass where
the channel is armored by the large boulders. The same conditions occurred in the Dajin Chuan and
Xiaojin Chuan rivers, which are also upstream tributaries of the Dadu River. At 17 km of the Xiaojin
Chuan and 20 km of the Dajin Chuan, respectively, upstream from the confluence, landslide dams created
two knickpoints, which effectively controlled the retrogressive erosion and channel incision of the two
rivers.
Large knickpoints at the margin of the Qinghai-Tibet Plateau range in elevation from 1,000 4,000 m.
The total water head of a large knickpoint can be as large as several hundred meters. The Hutiaoxia (i.e.,
Tiger Leaping Gorge) landslide and avalanche dam on the Jinsha River has a water head of 213 m.
Several large knickpoints at Hutiaoxia, Deqin, and Mangkang are shown along the longitudinal profile of
the Jinsha River in Fig. 3.49. The Hutiaoxia landslide and avalanche dam created a large barrier lake
(Fig. 3.50). Although the discharge of the river was so high to about 4,000 m 3 /s the water surface in the
barrier lake was as smooth as a mirror because of the huge depth and low flow velocity. The lake traps
all bedload and a large portion of the suspended load. The lake has not yet been filled. A step-pool
system with huge hydraulic jumps has developed downstream of the barrier lake, consuming most of the
flow energy and protecting the river bed from incision.
The large knickpoints may totally change patterns of fluvial processes and river morphology. The
upper Yellow River was an incised stream with many deep-gorge sections. Knickpoints developed from
landslide dams stopped the incision and formed high gradient reaches in the longitudinal profile. Figure 3.51
shows the bed profile of the upper Yellow River from the source (Erlin Lake) to Longyangxia Dam— the