Geoscience Reference
In-Depth Information
in that it allows greater understanding of the
processes that link to form the range of estuary
types (see section 7.1.2).
The term
delta
was first used by Herodotus
in 450 BC to describe the wedge-shaped land-
form, resembling the Greek letter delta (
water (lakes and quiet backwaters). Estuaries
and deltas do not readily fit into this category.
Here we have a unique and significant charac-
teristic of these environments. Estuaries and deltas
are areas where fresh and salt water meet. Fresh
water typically has a salinity of
),
seen at the mouth of the River Nile. A defini-
tion based purely on morphology, however, is
not useful when considering the range of delta
shapes that can occur. Barrell (1912) and sub-
sequently Bates (1953) altered this definition
to include some understanding of riverine pro-
cesses, before Wright (1982) combined the salient
points of the two and defined deltas as 'subaerial
and subaqueous accumulations of river-derived
sediment deposited at the coast when a stream
decelerates by entering and interacting with a
larger receiving body of water.' This highlights
the distinctiveness of deltas over estuaries, in
that the importance of high sediment volumes
and the accretion of sediment out onto the
coast are emphasized. In terms of both shape
and process, deltas, like estuaries, can be highly
variable, dependent on volumes of river water
and sediment, and the strength of marine currents
(see section 7.1.2).
Δ
0.5 NaCl,
whereas sea water is typically around 35. This
means that when the two mix to form brackish
water, the salinity will fall somewhere between
these figures, depending on the relative propor-
tions of salt and fresh water. The importance
of this is that the presence of salt enables clay
particles, through electrostatic attraction, to stick
together to form large sediment grains. This is
discussed in section 7.2.2, and is important
because in addition to clay particle adhesion, pol-
lutants can also be adsorbed, making estuarine
sediments potentially rich stores for pollutants
(see 7.2.3).
Sediment deposition occurs within estuaries
and deltas as a result of the complexities of
sediment delivery, current activity and marine
reworking. Over time, sediment builds up and
the surface becomes covered by tides for shorter
periods of time, i.e. the flood tide will not flood
the sediment surface until later in the tidal
cycle, and will leave it sooner during the ebb
tide. This means that ultimately, the sediment
surface can start to be colonized by vegetation.
This is fundamentally important in the future
survival of the developing marsh because: first,
root systems bind the sediment and help it resist
erosion; and second, leaves baffle the sediment-
laden waves and encourage more rapid sediment
deposition, thus further facilitating vertical marsh
growth.
Adam (1990) indicates that the first colon-
ization by salt marsh plants generally starts to
occur when inundation frequency falls below
c
. 500 times a year. However, the actual value
varies between estuaries. In the Blackwater
estuary, for example, colonization does not
start until inundation frequency falls below 380
times a year (Burd 1995). This initial vegetation
is salt tolerant (halophytic), because brackish
water will still flood over it once or twice a day.
As sediments build up further and tidal cover
is gradually reduced, less salt-tolerant vegetation
<
7.1.2 Nature and significance of deltas
and estuaries
Deltas and estuaries, because of their location at
the interface of rivers and seas, can be regarded
as major sinks and stores of sediment, particu-
larly terrestrial. In addition, the ability of waves
and tides to rework and shape these sediments
into recognizable landforms makes these envir-
onments very diverse in terms of morphology
and ecology. The sediment that forms deltas
and estuaries is also extremely variable, both in
terms of mineralogy (reflecting sediment source)
and grain size (clays to coarse grits). Consider-
ing that clays and silts are small sedimentary
particles, their deposition in these environments
of high tidal energy, waves and rapidly flowing
river water, is a considerable achievement. A list
of the depositional environments where clays
are found would largely involve situations where
sedimentation is possible from a still body of
