Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 119

3 Mountain Rivers and Incised Channels
Abstract
An incised river is defined as a river that is experiencing bed-level lowering. From the viewpoint of
geomorphological process, mountain rivers either were or are incised rivers. Large rivers may be incised
rivers in the upper reaches, but fluvial rivers in the lower reaches. The development of channel incision
in mountainous areas depends on the rainfall, watershed vegetation, and soil and rock compositions.
Incision may cause landslides, debris flows, and riverbed scour and in conjunction with bank erosion,
provides sediment to the flow. A step-pool system is a geomorphologic phenomenon occurring in high-
gradient mountain streams with alternating steps and pools. Cobbles and boulders generally compose the
steps, which alternate with finer sediments in pools to produce a repetitive, staircase like longitudinal
profile in the stream channel. The tight interlocking of particles in steps gives them an inherent stability
that only extreme floods are likely to disturb. Step-pool system maximizes the resistance, and, thus,
controls riverbed incision. A bedrock channel has bedrock exposed along the channel bed or walls for at
least approximately half its length, or has bedrock limits to the magnitude and location of bed scour and
bank erosion during floods. Bedrock channels most commonly occur in regions of high topographic
relief. Relief may be a product of recent tectonic uplift, as in the Himalayan Mountains of central Asia.
The exposed bedrock implies that the channels may be particularly sediment-starved during floods and
subjects to long term incision. The causes and evolution process of incised rivers and control strategies of
channel bed incision are also discussed in this chapter.
Key words
Mountain rivers, Step-pool system, Riverbed incision, Bedrock channel, Incision control strategies
3.1 Incised Rivers
3.1.1 RiverbedIncision
At the most fundamental level, without incision there is no channel. In a broad sense, therefore, one can
consider channel incision as a requirement of denudation, drainage-network development, and landscape
evolution (Darby and Simon, 1999). Channel incision has been and is a major concern of river managers
because it disrupts transportation, destroys agricultural land, threatens adjacent structures, drastically
alters environmental conditions, and produces sediment that causes further problems downstream. Therefore,
the causes of channel incision have been a topic of great interest because a better understanding of the
phenomenon could lead to prevention.
The most dramatic channel incision occurs on the east margin of the Qinghai-Tibet Plateau. Geologically,
the Indian Plate moves northward at a rate of 5 cm/yr and collides with the Eurasian Plate, resulting in the
uplift of the Himalaya mountains and the Qinghai-Tibet Plateau (Wikipedia, 2011). The Qinghai-Tibet
Plateau has become the highest plateau in the world and is referred to as the third pole of the earth. Uplift
of the plateau resulted in accelerated fluvial incision because of the remarkable increase in stream bed
slope at its margin. The fluvial incision into the plateau margin in response to tectonic motion resulted in
isolating remnants of the original plateau surface. These remnants of the plateau surface can be used as
reference surfaces against which to evaluate the impact of lateral erosion since uplift. The high loads of
sediment carried by the tributary streams of the Yellow, Yangtze, and Lancang rivers on the plateau
indicate rapid rates of denudation in the catchments.
Rapid fluvial incision into bedrock has been interpreted to reflect a tectonic uplift of similar magnitude,