Geoscience Reference
In-Depth Information
7
Deltaic and estuarine environments
Peter French
7.1
is between them. Both environments represent
areas of sediment accumulation at the coast, and
both are linked to the mouths of river systems.
The chief difference, however, is that estuaries are
features that are formed of marine and terrestrial
sediments within river mouths in response to
their flooding by a rise in sea-level. Deltas, on
the other hand, develop seawards of the coast-
lines where large volumes of fluvial sediment are
carried seawards, at a rate that exceeds the sea's
ability to erode it. Accepting this major distinc-
tion, however, there are many similarities, not
least of which is that the processes of tides, waves
and freshwater input that operate in estuaries
also occur in the distributary channels of deltas.
Therefore, a delta can be regarded as a coastal
landform composed of a series of outlets to the
sea that are, in effect, estuaries.
The term
estuary
originated from the Latin
word 'aestus' meaning tide (Woodroffe 2003).
The most commonly adopted definition was first
used by Cameron & Pritchard (1963), when they
referred to an estuary as 'a semi-enclosed coastal
body of water which has a free connection to the
open sea, and within which sea water is meas-
urably diluted with fresh water derived from
land drainage.' Although being a useful general
overview of what an estuary is, this definition
is generally regarded as an oversimplification as
there is no reference to the tidal processes that
are fundamental in shaping estuaries. E.C.F. Bird
(2000) amends this and defines an estuary as:
'the seawards part of a drowned valley system,
subject to tidal fluctuations and the meeting and
mixing of fresh water and salt water from the
sea, and receiving sediment from its catchment
and from marine sources.' This is more useful
INTRODUCTION
The focus of this chapter is those coastal environ-
ments that are associated with the mouths of
rivers: namely coastal deltas and estuaries. These
dynamic sedimentary systems occur at the inter-
face between terrestrial and marine systems and,
consequently, their sediment supply, morphology
and functioning are heavily influenced by both
land-based processes, such as river flow and flood-
plain development, and marine processes, such as
tides and waves. Also significant is the fact that
with the exception of lakes, these environments
receive all land drainage, inherent in which is
consideration of water volumes, sediment loads
and contaminants. This chapter examines these
environments within the context of how they
function as sedimentary systems, how they are
influenced by changes in natural processes, and
how they are impacted by anthropogenic activity.
Useful additional literature relating to estuarine
ecology (Adam 1990; McLusky 1989), the func-
tioning and physical aspects of estuaries (Dyer
1997) and estuarine management (French 1997)
are recommended. In terms of deltas, further
details relating to the processes of delta formation
can be found in many coastal geomorphology
textbooks (e.g. Woodroffe 2003), and discussion
of processes, because of the similarity between
estuaries and delta channels, falls within the liter-
ature cited above. Tropical mangrove-colonized
estuaries and deltas are referred to in Chapter 9.
7.1.1 Definition of estuaries and deltas
It is useful to discuss what is meant by the terms
'estuaries' and 'deltas', and what the difference
