Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 120

thereby sustaining topographic equilibrium. In uplifting mountain belts, an end-member scenario can be
formulated in which the rate of bedrock uplift is matched by the rate of stream incision and valley
lowering (Hovius and Stark, 2006). The slopes steepen until topographic elements become unstable and
collapse, producing rock falls, avalanches, and landslides. Tectonic uplift has been found to be
responsible for the abandonment of valleys, formation of deep and incised river valleys, highly irregular
longitudinal profiles of channels, and the varying number and tilting of terraces. Among these features,
deep valleys formed by the incision of rivers with high flow velocity are a prominent characteristic of
active, uplifting mountain areas. On a large scale, the ridge-valley landscape of the entire Qinghai-Tibet
Plateau was formed by the mechanism of intensive incision.
The continuous rising of the Qinghai-Tibet Plateau has resulted in steep slopes and several active
faults including the Longmenshan Fault where a great earthquake, known as the Wenchuan Earthquake,
occurred on May 12, 2008. River-bed incision has dominated the fluvial process in the area. The frequent
landslides on threshold hill-slopes is one means of relief adjustment to fluvial bedrock incision. Besides
the rate of rock uplift and incision intensity, the characteristics of the underlying rock are another
controlling factor. Limestone usually has great permeability, thus allowing rainfall to infiltrate to the
subsurface and reducing incisions on the surface (Hovius and Stark, 2006). Rocks with lower permeability,
but vulnerable to weathering, usually are more prone to mass wasting; for example, the granite in the
lower Minjiang River. Observations have revealed that steady incision during low and intermediate flow
conditions leads to channel bed lowering while significant channel widening occurs during big floods.
Crucially, such floods help transmit the effect of the accumulated thalweg lowering to adjacent hill slopes.
Deep landscape dissection has produced high-relief, narrow river gorges, and threshold hill-slopes that
frequently experience large landslides, making the entire region highly susceptible to quake lake formation.
The quake lakes and their management play an important role in the morphological process and reclamation
of the land in the earthquake area.
Of course, channel incision occurs mainly in mountainous rivers. However, incision may also occur in
channels of relatively gentle slope. Because of dense population and infrastructure channel incision in an
in habited plane area is more disastrous, although the incision is in much smaller scale than that on the
Qinghai-Tibet Plateau margin. For example, rapid urbanization and the resulting increase in peak flows
from a watershed can result in substantial channel incision in urban areas threatening homes, roads, and
other infrastructure, serious incision problems have resulted in the bluffs near Lake Michigan in the
Chicago and Milwaukee areas in the U.S. Release of clear water from a reservoir also may cause incision
of the downstream reaches, and channelization of a river may cause continuous scour of the channel bed
(Wang, 1999). The Sanmenxia Reservoir began filling with water in 1960 and released clear water beginning
in September 1960. A 400 km long reach was scoured when the reservoir released water at a rate of
1,000 m 3 /s and an 800 km reach along the riverbed was scoured when the released discharge was over
2,000 m 3 /s (Yang et al., 1994). From September 1960 to October 1964, the released clear water from the
reservoir scoured 2 billion tons of sediment from the bed of the lower Yellow River. The incision of the
riverbed endangered bridge piers and exposed oil pipelines, which were buried beneath the riverbed. The
Dashi River is located to the north of Qinhuangdao, with bed sediment mainly composed of sand and
gravel. An oil pipeline from the Daqing oil field to Qinhuangdao crossed the river. A 1,126 m long pipe
section was buried 2 m beneath the bed. A reservoir with a storage capacity of 70 million m 3 was built
3 km upstream of the river-pipeline crossing. The reservoir released a discharge of 4,250 m 3 /s, equal to
the 100-year flood, in the summer of 1984. The riverbed was seriously scoured and the oil pipe was
exposed to the flow and was broken. A large amount of oil was discharged into the river, which resulted
in serious environmental pollution.
The middle reaches of the Mississippi River were shallow and unstable. American engineers began to