Integrated River Management - River Dynamics and Integrated River Management - page 731

the river have regulated the flow and trap sediment load, thus, greatly changing the flow and sediment
load. The Lijin station is the most downstream station on the Yellow River. The runoff and sediment load
measured at the station represents the amounts of water and sediment transported into the sea. As shown
in the figure, both runoff and sediment load have substantially reduced since the 1970s. The reductions of
runoff and sediment reduce the flooding risk, wandering rate of the channel, and management difficulties.
Nevertheless, the reduction in water flow has caused water shortages in the lower reaches, which have
been discussed in Chapter 6.
It should be pointed out that gradual reduction of sediment transportation in rivers may improve the
river health, but abrupt reduction may also cause stress on river management, land creation, and ecology.
Some river training projects have aimed to enhance sediment-transport capacity, which is against
Principle ē . If sediment transportation in a reach is intensified, local sedimentation may be controlled but
an abnormal stress is put on the downstream reaches. Moreover, high sediment-transport capacity can
substantially fluctuate and is difficult to manage. In the Ming Dynasty, Pan proposed the strategy of
narrowing the lower Yellow River and confining floods within the stem channel between the Grand
Levees in order to raise the velocity and maintain a high sediment-carrying capacity, to prevent sediment
from depositing and even promot bed sediment scouring (details are given in Chapter 6). He regulated
the levee system, blocked many branches of the river and made the river flow in a single channel in the
period 1565-1592. Nevertheless, the strategy of narrowing the channel was difficult to apply and soon
after Pan's projects the sediment deposition in the downstream channel sped up to 5-10 cm per year.
Principle Ĕ —Increasing Diversity and Connectivity of Habitats
The streams provide habitat for benthic macro-invertebrates and fish. The larger is the habitat area, the
more species living in the habitat. According to research results, biodiversity is directly proportional to
the stability and diversity of the habitats. The river management should increase the habitat diversity. To
increase diversity two aspects must be met: (1) increase the water surface area in the river course and
riparian waters and the connectivity between water bodies; and (2) increase the water area with low
velocity and varied water depths, such as bays and lakes.
Some river training works result in the fragmentation and isolation of habitats. Figure 11.13(a) shows
the concrete banks of an urban channel in Beijing, which have a bad effect on the ecology. The aquatic
creatures living in the sediment bed lose their shelter and thus have disappeared from the channel. Only a
very tolerant species of mosquito, which may survive in seriously polluted water, is found in the channel.
Recently the Beijing government has decided to abandon the concrete channel bed and banks and to use
the sediment bed and stone and vegetation banks instead for ecological improvement. It is reported that
after the concrete banks and bed were replaced by a sediment bed and stone and vegetation banks the
water volume needed to maintain good water quality has been halved.
Linkage of riparian wetlands with rivers is important for ecology. Fig. 11.13(b) shows a river-linked
wetland by the Zengjiang River. The Zengjiang River is a tributary of the East River in south China.
Substrate in the main channel of the river consists of quartz sand. No benthic invertebrates were found in
the samples taken from the river bed because the sand bed is unstable. The wetland by the Zengjiang
River is like a riparian lake with a 100 m wide outlet connecting the river. The river carries fine suspended
sediment into the wetland which is deposited there. A mud layer covers most of the wetland, and some
hydrophytes have colonized parts of the wetland. The flow velocity and water depth in the wetland vary
in the range of 0-0.5 m/s and 0-3 m, respectively. The wetland provides multiple habitats for benthic
invertebrates. The taxa richness in the sample taken from the wetland is 31, and the abundance of
individual invertebrates is 343 ind/m 2 . Many fish species are found in the wetland.
Dam construction and hydro-power exploitation cut off the migration of fishes from downstream to
upstream for spawning and impair the ecology. Fish ladders have been built on many large dams to