Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 37

Reforestation is effective in wet areas (the southern China, for example) if mass movement of soil is
controlled by multiple check dams. The success of reforestation relies more on the agricultural policy
than technology. The change of ownership from community to private households has incited incentives
for farmers toward reforestation since the 1980s. After the 1998-flood on the Yangtze River state-funded
reforestation projects in the upper Yangtze River watershed sped up. Reforestation has been successful in
many watersheds. Nevertheless, in mountainous areas people are still burning wood for cooking and
heating. Planting shrubs and fast-growing trees in selected zones to provide the local people fuel wood is
an effective measure to protect the forest.
1.2.4 Riverbed Incision and Geological Disasters
Riverbed incision is defined as continuous bed erosion and bed-level lowering. Mountain rivers either
were or are incised rivers. Alluvial rivers may also experience a shot period of bed incision. Riverbed
incision may be caused by tectonic motion, meanders cutoff, stream caption, dam construction, channelization
and many other causes. Geologic and geomorphic causes require many years to develop a response,
whereas climatic and hydrologic variability, animal grazing, and human activities can have a more immediate
impact. Figure 1.31(a) shows the stream bed incision in the upper Yangtze River (Jinsha River) in the
southwestern China. The rising Qinghai-Tibet Plateau increases the stream slope and bed erosion. The
plateau has deeply incised by the river by more than 2,000 m. Figure 1.31(b) shows an incised section of
the Yunzhong River, which is a tributary of the Hutuo River in Hebei Province in North China.
Impoundment of a reservoir trapped sediment and released clear water, which resulted in several meters
bed incision in the downstream reach of the reservoir.
(a) (b)
Fig. 1.31 (a) The upper Yangtze River in the southwestern China incised the Qinghai-Tibet Plateau by more than
2,000 m; (b) Impoundment of a reservoir on the Yunzhong River caused channel bed incision in a downstream reach
by several meters
Almost all geological hazards are associated with river bed incision. Channel bed incision increases
bank slope and causes bank failures, landslides and debris flows. Moreover, bed incision of a river
propagates to its tributaries and gullies. Finally all slopes become steeper and more serious soil erosion
occurs. A persistence of landslide events occur in the actively incising river gorges on the eastern margin
of the Qinghai-Tibet Plateau. The direct causes of the landslide events were rainstorm and earthquake,
while the essential cause of the events was river bed incision. The Wenchuan earthquake on May 12, 2008
triggered several hundreds of large scale landslides on numerous rivers, which were all incised rivers.
Figure 1.32(a) shows a huge avalanche on the deeply incised Kangding River on the east margin of the