Estuary Processes and Managment - River Dynamics and Integrated River Management - page 461

less than 0.01%. The area of their deltas is very large, in a range of 10-10,000 km 2 . The classification of large
river deltas is different from that of small rivers. The dynamic, morphologic and ecological characteristics
of the deltas are very different and can be classified into two types: male deltas and female deltas. In general,
male deltas are not stable and have only one or two channels. Female deltas consist of complex channel
network and numerous islands, which are relatively stable.
Male delta —Male delta develops if a river carries high sediment load and the tidal currents are weak.
Sediment-laden rivers create land at high rate and extend into the ocean, forming a fan-shape delta. Such
a process is accompanied with periodic nodal avulsions. The Yellow River carries sediment from the
loess plateau in central China to the delta and causes the delta to expand by 2,000-3,000 ha per year. The
present Yellow River delta has developed over the past one and half centuries as a consequence of
channel siltation, flooding, and avulsions. Human activities have also changed the delta processes. Oil
fields and a new city with a modern infrastructure have been developed on the newly created land. The
river channel within the delta is unstable and avulsions often occur. The river mouth extends into the sea
at a rate of about 2-3 km/yr, depending on the incoming sediment load. The details of the development
history of Yellow River delta is described in Chapter 6. The extension of the river channel reduces the
gradient and the sediment-carrying capacity, which results in avulsions occurring. In general, the length
of the new channels are about 1/3-1/2 of the previous ones and the gradient is 2-3 times higher (Wang et
al., 2003). Figure 8.1 shows the Yellow River delta in 1996, which had extended into the Bohai sea for
more than 40 km in 20 years since the avulsion from the Diaokouhe channel to the Qingshuigou channel
(Zhang et al., 1999).
Fig. 8.1 Yellow River delta is male, which has one or two channels and the channels are not stable
The Mississippi River in the U.S., the Luanhe River in China, the Ebro River in Spain and the Ural River
in Russia are also male rivers. The basic features of these rivers are given in Table 8.1. The Mississippi
River mouth extends deeply into the Mexico Gulf. Avulsions occurred in the Mississippi delta along the
coast of Louisana as successive channels and searched for gradient advantages over their precursors (Leeder,
1983). The delta development is affected or partly controlled by humans. Upstream reservoirs, changes
in agricultural practices and land uses, and bank stabilization measures have reduced average sediment
loads in the lower Mississippi River by approximately 67% since the 1950s (Melching, 2006). The average
concentration of suspended sediment has been reduced from 0.8 kg/m 3 in 1950 to 0.24 kg/m 3 in 2000.