Geoscience Reference
In-Depth Information
the relative contribution of each to the sedi-
ment budget. The reworking of material from
within an estuary or delta (endogenic) is more
likely during the more stormy winter months,
or, in the longer term, during episodes of sea-
level rise. In terms of biogenic material, both
plants and animals can make a contribution.
Plants are perhaps the greatest component of
this, adding dead plant material to the marsh
surface and representing an important element
of marsh vertical growth (see section 7.6.1),
although inputs are seasonally variable. Animal
material is more variable. Although, in principal,
skeletal material is potentially a large contribu-
tor (Frey & Basan 1978), it is argued that in
reality actual amounts present in a sediment
sequence are typically small because the durabil-
ity of shell material is low, particularly when
acidic conditions develop in the marsh sediment
sequence (Wiedermann 1972). Faecal material
is also an important contributor to organic sedi-
ment (Frey & Basan 1978).
Microscopic organisms such as diatoms and
algae fill two roles. First, they contribute
post
mortem
to the volume of organic remains in
the sediment; and second, they play a key role
in sediment stability. Diatoms and algae secrete
carbohydrate-rich exopolymers, which serve
to stick fine sediment grains together, thus
increasing their resistance to erosion. Dyer
(1998) demonstrates the importance of this
process in the general salt marsh context, and
Underwood & Smith (1998) in the Humber
(UK), Kornmann & de Deckere (1998) in the
Dollard (The Netherlands), and Riethmüller et
al. (1998) in the Wadden Sea (The Netherlands-
Danish Coasts); all demonstrate the importance
in particular environments.
Input from outside the system (exogenic)
is more likely to occur when soils are bare of
vegetation or during periods of catchment land-
use change. For example, increased sediment
supply as a result of forest clearance in the catch-
ment of the Mahakan delta, Kalimantan caused
rapid delta growth (E.C.F. Bird 2000), and
mining operations led to accelerated sediment
supply and subsequent delta growth in the
George River delta in Tasmania ( J.F. Bird 2000),
the Pahang delta in Malaysia (E.C.F. Bird 2000)
and the now disappeared Fal delta (UK) (Bird
1998). Conversely, reduced sediment supply
can lead to reduced marsh or delta growth. The
erosion of the Nile delta caused by the building
of the Aswan High Dam is perhaps the classic
example of this (see Case Study 7.2). Other
examples where reduced river flow has caused
a reduction in sediment supply include the
Rhône delta, France, the Dneiper delta, Ukraine
and the Barron delta, Australia (E.C.F. Bird
2000). Although the majority of the examples of
reduced sediment supply are linked to dams and
other water-control measures, other causes also
exist. River dredging can remove large quantit-
ies of sediment from the local sediment budget,
the cessation of mining activities can remove
artificially high rates of sediment supply, and
sediment increases following land disturbance
can effectively 'run out'. As an example of the
latter, the Argentina River delta, Italy, started
to erode following reduced sediment supply
after having initially experienced accelerated
accretion as a result of increased soil-derived
sediment inputs from land clearance. The sub-
sequent erosion of this soil from the hinterland
and the laying bare of the landscape to bedrock
led to a cessation in sediment supply (Bird &
Fabbri 1993).
In conclusion, the key factor for ensured
stability is that whatever sediment the estuary
or delta loses, whether via erosion to the sea or
via decreased inputs, it is balanced by newly de-
posited sediment, i.e. a positive sediment budget.
In the case of deltas, the continued growth of the
delta, and seaward extension of the delta front,
is dependent on the continued delivery of sedi-
ment from the hinterland. Similarly, the contin-
ued vertical accretion of intertidal flats and salt
marshes is dependent on the accumulation of sedi-
ment on its surface to replace that removed by
the tide and to compensate for rising sea-levels.
7.2.2 Controls on sediment accumulation and
transport in deltas and estuaries
The sediment inputs described above need to be
linked with mechanisms by which they can be
