Geoscience Reference
In-Depth Information
reservoirs for drinking, irrigation or other pur-
poses, hydroelectric power, removal of natural
vegetation and waste disposal (Newson 1992).
Typically, regulation is carried out by dam and
reservoir building, and channelization, which
can involve resectioning, sediment extraction,
the use of flood walls, culverts, embankments or
levees, and straightening.
Sedimentation in regulated rivers can be con-
siderably different from that in natural rivers.
In dammed rivers, reservoirs act as sediment
traps, significantly affecting rivers with medium
to large natural sediment loads. Downstream of
dams, many rivers have been reported to supply
less sediment after damming for irrigation, flood
control and power generation. This reduction
has resulted in catastrophic changes in down-
stream river reaches, and deltas and coastlines,
which are often eroded as a result of increased
stream power (Trenhaile 1997). In the Missouri
and Mississippi rivers, for example, the reduction
of annual sediment yield from 817 million tonnes
to 204 million tonnes in the twentieth century is
thought to result from large-scale dam projects.
This loss in sediment yield is in turn linked to
rapid coastal degradation and recession in down-
stream Louisiana (Keown et al. 1986). Walling
& Feng (2003) carried out an extensive study of
long-term records of annual sediment load and
runoff for 145 major rivers, and demonstrated
that although c . 50% of the rivers experienced
statistically significant upward or downward
trends, most showed the latter. This was largely
attributed to dam construction. Impacts of the
Aswan High Dam on the River Nile are also dis-
cussed in Chapter 7, and the Hoover Dam in
Colorado in Chapter 1.
Many studies have demonstrated effects on
river sedimentation as a result of river regulation
(e.g. Case Study 3.1). Arnaud-Fassetta (2003)
Case Study 3.1 Past, present and future impact of the Three Gorges Dam on the Yangtze River
The Yangtze River (Changjiang) is the longest river in Asia (
>
6300 km), the third longest
10 6 km 2 ) in the world. Its water
discharge is the largest in the western Pacific Ocean, and its sediment load (annual discharge of
480
in the world and has the ninth largest catchment basin (1.8
×
10 6 tyr −1 before the 1990s) is the third largest in the world behind the Amazon and the
Congo. Shanghai, the most important industrial and economic city of China, is located near the
river mouth. The 39.3
×
10 9 m 3 Three Rivers Gorge Dam project, scheduled to be completed
between 2009 and 2012, will be the largest dam in the world, with estimated power production
equalling approximately 12 nuclear power plants. The dam is being built for flood manage-
ment, inland navigation and hydropower.
Yang et al. (2002) reported that decadal river sediment discharge and suspended sediment
concentrations (SSC) had reduced by 34% and 38%, respectively, between the 1960s and 1990s
(Case Fig. 3.1a). This was in contrast to the period between the 1950s and 1960s, when annual
sediment discharge and SSC increased by 10 and 12%, respectively. Yang et al. (2002) attributed
these increases to deforestation, and the 1960s to 1990s decreases to a combination of dam and
reservoir construction (Case Figure 3.1b). The authors used these data to predict that both river
sediment discharge and SSC will probably decrease to 40% of original levels in 50 years and to
50% in 100 years, largely as a result of the dam and reservoirs. Critics have suggested that the
damming will also lead to the build-up of sediment-borne contaminants, damage of turbines by
sediment, increases in deforestation and soil erosion as former inhabitants relocate and erosion
in the delta downstream of the dam. Wang et al. (2005) have demonstrated that, after the com-
pletion of the dam, anastomosing channels downstream of the site will cease to carry significant
amounts of water and sediment, and thus disrupt the balance between fluvial discharge and
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