Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 36

in which the amount of nitrogen, phosphorus and potassium in the eroded soil is about 50 million tons,
which is much higher than the annual national production of fertilizer (30 million tons). The farmland
deteriorates and grain production also is reduced by erosion. Moreover, the nutrients are transported with
the sediment into the environmental waters and causes eutrophication and red tide. More than 240
harmful algal blooms occurred in the 1990s, causing huge economic losses. Erosion exerts the highest
ecological stress on the vegetation cover. In the northern part of the Loess Plateau, the vegetation can
hardly develop because the extremely intensive erosion carries off the topsoil on which the vegetation
relies. In the areas with vegetation, such as the upper reaches of the Yangtze River, erosion damages and
destroys the vegetation and scars the land surface.
Erosion from the upper reaches provides the rivers with too much sediment that causes sedimentation
of the channels and high flood risks. For instance, the Yellow River is notorious for its frequent avulsions
and flooding disasters. The main reason is the heavy sediment load from the erosion of the Loess Plateau.
The Yellow River sediment causes not only the siltation of the lower channel reaches of the river itself,
but also the shrinkage of the mouths of drainage channels of the Haihe River drainage system. Figure
1.30 shows that the silt drifting from the Yellow River mouth under the action of tidal currents is deposited
at the New Ziya River mouth, which is one of the major drainage channels of the Haihe River Drainage
System. The channel is silted up by 6 m and the discharge capacity of the channel has reduced by 60%
(Hu et al., 1999).
The main erosion control strategies applied in China are building sediment barriers in gullies and
terracing the sloped farmland in the arid and semi-arid areas; building sediment-check dams and reforesting
the hills in the wet areas; and comprehensive reclamation of small river basins in both arid and wet areas.
Comprehensive reclamation of small basins is the main strategy applied in the Loess Plateau. The area is
arid and semi-arid and, therefore, reforestation alone is difficult to achieve the objective of erosion control.
People built many sediment barriers and created productive warped farmland. Terraced fields enclosed
with borders 20 cm high may trap almost all rainfall water and greatly reduce erosion. Impounding water
with dam on rivers provides water for drinking, irrigation, and reforestation. People plant grass on the
slope and trees around the fields and roads. As a result, the sediment fed into the rivers by erosion from
the Loess Plateau has been greatly reduced since 1984 (Gu, 1994).
Fig. 1.30 The silt drifting from the Yellow River mouth under the action of tidal currents is deposited at the New Ziya
River mouth. The channel is silted up by 6 m and the discharge capacity of the channel has reduced by 60% (See color
figure at the end of this topic)