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In-Depth Information
For example, lobate deltas may have distinct cusp-
ate traits facing the prevailing wave fronts, yet
not elsewhere (see Woodroffe (2003, pp. 324 -5)
for a discussion). The appearance of the term
'compound deltas' may have helped in this respect,
but has tended to become a bucket term, where
anything not conforming to the easily identifi-
able is placed. Further complicating the issue is
the fact that many deltas change over time as
they mature and as processes such as river input,
continental shelf topography, or marine processes
change. For example, the history of the Mississippi
delta shows such a pattern (Coleman 1988) (see
also section 7.3, Fig. 7.11). Initially, sediment
lobes form (lobate trait) and undergo enlarge-
ment by seaward progradation. This lobate stage
is followed by the development of a system of
distributary channels (birdsfoot trait), which over
time will switch and develop into a complex
birdsfoot pattern. The final stage is for this active
delta front to be abandoned and for a new lobate
front to form elsewhere in the delta system. In
the Mississippi, there are six major lobate to
birdsfoot cycles, with a typical periodicity in the
order of 1500 years (Coleman 1988).
Despite this complication, as with estuaries,
different forms of delta classification may suit
different needs. A shape-based approach, as in the
Wright and Coleman model, remains convenient
for morphological classification needs (Fig. 7.2).
If studying how a delta functions in respect to
sedimentation and process, however, other forms
of categorization may be better suited. In this
respect, the system suggested by Bates (1953) is
useful as it is linked to processes, and in particular
density differences, between the inflowing water
and the relatively still receiving body. Where the
inflowing river water is denser than the receiving
water, the system is referred to as hyperpycnal,
where they are the same, homopycnal, and where
the river water is less dense, hypopycnal.
Agricultural
runoff
Urban runoff
Marine
sources
Industry/ports
and harbours
Water
body
Younger
Atmospheric
dust
Wetlands
Older
Intertidal
deposits
Subtidal
deposits
Fig. 7.3
The range of potential inputs to estuaries and deltas
(water body). Note that some of these inputs are one way,
whereas others represent a bi-directional exchange.
tion of terrestrial and marine sediments. From
other chapters, it is evident that terrestrial- and
marine-derived sediment may originate from
different areas: for example, from upland areas
via rivers (Chapters 2 & 3), from urban runoff
(Chapter 6), or from the coast (Chapters 8 & 9)
and offshore (Chapter 10) (Fig. 7.3).
7.2.1 Sources of deltaic and estuarine sediments
Figure 7.3 highlights the various potential
inputs to estuaries and deltas. Actually quantify-
ing these parameters is notoriously difficult
owing to problems of measurement and access-
ing the more remote parts of the system, such
as the incoming tide. In total, however, these
components comprise the sediment budget.
Basically, this is a summation exercise in which
all of the inputs are totalled and offset against
the losses. The resulting value, if positive, will
indicate an estuary or delta with a net sediment
gain, whereas if negative, it will indicate one
with a net sediment loss.
In terms of source, the various inputs iden-
tified in Fig. 7.3 can be grouped as exogenic and
endogenic, depending on whether they origin-
ate outside the system (from the catchment or
sea) or within (through reworking of existing
deposits or generated within the estuary through
biogenic activity). Regardless of source, however,
there will be considerable temporal variation in
7.2
SEDIMENT SOURCES AND SEDIMENT PROCESSES IN
DELTAS AND ESTUARIES
The source of delta sediments is predominantly
terrestrial, whereas estuaries contain a combina-
