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quantitative modeling of delta evolution, stratigraphy
(Fagherazzi and Overeem
2007
) , and clinothem devel-
opment (Swenson et al.
2005
; Slingerland et al.
2008
) .
One continuing challenge, though, is the diffi culty in
numerically modeling tidal sediment transport due to
complications of the bidirectional fl ow, thus limiting
our ability to assess impacts of environmental changes
such as discharge variations, sediment loading, and
sea-level change. Although effective modeling of tidal
sediment transport remains elusive, progress is being
made in understanding hydrodynamics of the complex
network of tidal channels (Fagherazzi
2008
) and com-
pound clinoform morphology (Swenson et al.
2005
;
Wright and Friedrichs
2006
) that characterize tide-
dominated delta systems. These topics are discussed in
detail later in this chapter.
continental shelves and seas that are well connected to
the open ocean, and in many instances taper in width
toward their apex. Prominent examples include the
Arabian Sea (Indus), Bay of Bengal (Ganges-
Brahmaputra), Andaman Sea (Ayeyarwady), Gulf of
Papua (Fly), and East China Sea (Changjiang). A sec-
ond factor common to most tide-dominated deltas, and
many deltas in general, is that they drain high-stand-
ing, tectonically active mountains. Such active orogens
yield the abundant sediment required for deltas to form
in high-energy coastal basins. In particular the
Himalayan-Tibetan uplift and Indonesian archipelago
sustain among the world's highest sediment yields
(Milliman and Syvitski
1992
) .
7.2.3
Humans and Deltas
7.2.2
Modern Examples
Many tide-dominated deltas are among the world's
largest in areal extent (Woodroffe et al.
2006
) , and the
immense, agriculturally rich, lowland delta plains that
have formed at the mouths of the Ganges-Brahmaputra,
Indus, Ayeyarwady, Mekong, and Changjiang rivers
support nearly 200 million people. These populations,
like those in all deltas, are at risk from fl ooding, tropi-
cal cyclones, sea-level rise and related environmental
hazards. Unfortunately, our current understanding of
the process-response (morphodynamic) relationships
in tide-dominated deltas is inadequate to assess the
likely outcome of various environmental-change sce-
narios. Much may be learned by further investigation
of the several tide-dominated deltas that have already
been severely degraded due to river damming, water
extraction, and reduced sediment discharge, notably
the Indus, Colorado, and Tigris-Euphrates (Syvitski
et al.
2009
). Despite risk and uncertainty, major dams
continue to be constructed on rivers that feed high-
energy, tide-dominated delta systems, such as the
Three Gorges Dam on the Changjiang and the Xiaowan
Dam on the Mekong (Yang et al.
2006
; Kummu et al.
2010
). Not all tide-dominated deltas are strongly
human-impacted, however, with the Amazon, Copper,
and Fly river systems draining relatively natural catch-
ments and having sparsely populated delta plains.
Similarly, the Ganges-Brahmaputra and Ayeyarwady
rivers remain undammed despite their heavily popu-
lated catchments, and so their large water discharge
and sediment loads sustain stable, if still locally
dynamic, delta systems.
In this chapter we focus primarily on tide-dominated
deltas, including examples of the Colorado, Fly,
Ganges-Brahmaputra, Indus, Irrawaddy, and
Changjiang, with some discussion of tide-infl uenced
deltas such as the Amazon, Mahakam, and Mekong.
Overall these systems are best characterized by their
wide river mouths that have a pronounced upstream
taper and well-developed channel bars and islands. All
examples are subject to mesotidal to macrotidal condi-
tions with spring tidal ranges typically ³3 m. Because
of this continual exposure to tidal exchange and sedi-
ment transport, tide-dominated deltas along open shore-
lines are typically fed by large rivers that discharge
high sediment loads, although smaller rivers may form
deltas in more embayed settings (e.g. Gironde River,
France). Indeed, 10 of the river deltas listed above
(excluding the Mahakam) rank among the world's top
25 rivers in terms of their fl uvial sediment discharge
(Milliman and Meade
1983
; Milliman and Syvitski
1992
). Rankings for the Colorado, Tigris-Euphrates,
and Indus rivers are based on historical estimates prior
to major damming and sediment trapping.
Most tide-dominated deltas today are located in tec-
tonically active, low-latitude regions, including South
Asia, East Asia, and Oceania (Fig.
7.1
). Many factors
relevant to the development of tide-dominated delta
systems are common to these areas. First, amplifi ca-
tion of the M2 tidal component in high tidal-range
areas is supported by broad, relatively shallow
