Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 18

reach are separated by vegetated islands. The islands are composed of gravel, sand and soils. Although
not as frequent as single-thread channels, braided channels occur in a wide range of environments, from
proglacial to semi-arid and at large range of scales, from the small streams on sandy beaches to the largest
continental rivers.
(a) (b)
Fig. 1.15 (a) Braided channels of the Lhasa River on the Qinghai-Tibet Plateau; (b) Anabranching channels of the
Minjiang River, the famous Dujiangyan irrigation project is shown in the picture
Wandering rivers —A wandering river is defined as a river with an unstable channel. A wandering
river carries a heavy sediment load and the discharge and sediment-carrying capacity are unsteady. A
wandering river is usually associated with river aggradation. There are sand bars in the river channel but
usually the stream flow remains in one channel during one period of time and flows in another channel
during a second period of time. This is different from the braided rivers, in which the stream flows in
multiple channels simultaneously. One important feature of a wandering river is the high speed of migration
of the main stream channel.
The lower Yellow River is a wandering river with bed material mainly composed of silt and fine sand
of median diameter of about 0.02 mm. The sediment is very erodible. Bank erosion occurs at one side,
meanwhile sediment deposition occurs at the other side of the channel. Figure 1.16 shows the wandering
motion of the Yellow River section upstream of Zhengzhou in the periods of 1960-1964 and 1981-1985
(Wang et al., 2001). In the period of 1980-1985, hyperconcentrated floods occurred and the river channel
wandered at high speed and the riverbed was silted up. At the Luocunpo cross section, the stream channel
moved southward more than 5 km in the period of 1984-1985.
Anastomosing river —Schumm (1968) first introduced the concept of anastomosing river for describing
multiple channel system with straight, meandering or braided reaches. Knighton and Nanson (1993)
described it as a system of multiple channels separated by islands, which are usually excised from the
continuous floodplain and which are related to the size of the channels. Smith and Smith (1980) used the
term “anastomosing river” for an interconnected network of low gradient, relatively deep and narrow,
straight to sinuous channels with stable banks composed of fine grained sediment (silt/clay) and vegetation.
Wang (2000, 2002) analyzed the characteristics of anastomosing rivers and indicated that anastomosing
river is a result of development of channels on floodplain and braided river is a result of development of bars
and small islands on rivers. Ni et al. (2000) studied the transformation of anastomosing river and braided
river or meandering river. However, anastomosing river has not got its position in the conventional
classification of river patterns. In fact, many Chinese rivers, especially in northeastern China, are
anastomosing river. Figure 1.17 shows a satellite image of the anastomosing Mudanjiang River in