Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 127

a main channel can cause a local steepening of the channel gradient and incision. It has been demonstrated,
for example, that sediment deposition and storage in valleys and on alluvial fans eventually leads to
steeper gradients and the formation of discontinuous gullies (Fig. 3.1) and alluvial-fan trenches (Schumm
et al., 1984), when a threshold of slope stability or stream power is exceeded. Cliff retreat and lateral
channel shift can shorten and, therefore, steepen the affected channels.
Other processes, such as mass movement, can produce incision. Mudflows and debris flows are
capable of erosion and channel enlargement, and when a landslide delivers large amounts of sediment to
a valley floor, this deposit will eventually be eroded and become incised by the existing stream. In
addition, where groundwater emerges from a sloping surface, a channel can be produced that extends
back toward a drainage divide or to the source of the groundwater.
3.1.2.3 Climatic and Hydrologic Causes
Climatic and hydrologic causes of incision are closely related, and perhaps they should not be separated
in Table 3.2. Nevertheless, it is possible to think of a climate change that alters vegetation. A change to a
drier climate can reduce vegetation cover and produce higher sediment loads and higher peak discharges.
A change to a wetter climate can increase vegetative cover, which reduces sediment loads and increases
mean annual runoff. These climatic changes produce hydrologic changes that cause incision. Climatic
fluctuations also may produce hydrologic responses, such as major floods, periods of increased discharge,
and changes in sediment loads, without greatly affecting vegetative cover.
3.1.2.4 Human and Animal Causes
Human activity of a variety of types is known to cause channel incision. These causes can be grouped
into four effects as follows: decreased sediment loads, increased annual discharge and peak discharge,
flow concentration, and increased channel gradient. Decreased sediment loads can be caused by dam
construction, sediment mining, urbanization, diversion of sediment into another channel such as a canal,
and gravel mining and dredging. Flow can be concentrated with the effect of increasing stream power by
gravel mining and dredging, urbanization, roads, trails, ditches, channelization, and by flow constriction
by dikes. Gradient can be increased by dam removal, lowering of water levels in lakes and reservoirs,
meander cutoffs, mining, withdrawal of fluids and hydro-compaction, gravel mining, dredging, and
channelization.
Figure 3.6(a) shows sediment mining from the Lishui River in Hunan Province, China. Sediment
demand for building materials has been increasing due to economic development and urbanization in
China. There are no laws to control sediment mining from rivers. Consequently, the total amount of
sediment mining in many places is much more than the annual coarse sediment load (bed load). As a
result the channel bed has incised down by several to twenty meters.
Channelization has become an important cause of bed and bank erosion. Figure 3.6(b) shows that
channelization of the Charlooz River in Iran causing channel incision, which endangered the banks and a
nearby road. The banks are hardened and smoothened, therefore, high velocity current may flow near the
banks and cause erosion of the bank toe. The river channel is narrowed and straightened because of
urban construction. The flow velocity is enhanced, which caused river bed incision and bank collapse, as
shown in the figure.
It has been argued long that overgrazing has produced the great arroyos of the southwestern U.S.
Whether or not this is true, weakening of vegetation cover by grazing can produce channel incision.
Tracking of animals also decreases infiltration rates, which in turn increases runoff both on hill slopes
and in the trails.