Geoscience Reference
In-Depth Information
Table 7.2
Human activities and related impacts in estuary and delta environments.
Activity
Impact
Land claim
Loss of intertidal habitat, coastal squeeze, increased need for flood defences
Coastal defence
Loss of intertidal habitat, increased erosion, changes to natural sediment cycling,
coastal squeeze, alteration to wave and tidal processes
Tourist development
Increased visitor pressure, habitat loss, increased contamination /pollution,
increased need for coastal defences
Industry
Contamination /pollution, habitat loss, increased need for defences, increased need
for shipping access (dredging)
Barrages
Loss of intertidal habitat, changes to tidal regime, contaminant retention, greater
brackish water penetration up river, sediment retention, changes to water table
Sediment extraction
Loss of submarine habitat, changes to tidal and wave currents, loss of wave
protection, re-suspension of sediment and remobilization of pollutants
Waste disposal
Contamination /pollution, increased turbidity
Dams
Reduction in freshwater supply/increase in net salinity, reduction of sediment supply
from catchment
Catchment land use changes
Changes in sediment supply, changes in pollution levels, changes in freshwater input
their suitability for industrial and agricultural
development. Human activity has, therefore, pro-
duced significant modification to many systems
(Table 7.2).
dence times for pollutants in estuarine channels,
and sediment deposition patterns were modified
by weakened ebb currents (Collins & Evans
1986). The only sediment that now reaches this
delta comes from erosion of sediments down
stream of the dam. Dams on the Ebro (Spain)
and the Rhône (France) have also reduced sedi-
ment supply to coastal deltas by 96% and 90%
respectively (Viles & Spencer 1995).
As well as affecting the amount of sediment
entering the estuary or delta, dams also retain
fresh water, thus reducing the quantity that
enters the estuarine area. Stratified and partly
stratified estuaries (see Fig. 7.1) rely on fresh
water to drive their circulation and create the
stratification. This freshwater-induced circula-
tion also controls the movement of nutrients on
which primary and secondary productivity of
the system depends (Mann 2000). More funda-
mentally, many brackish water species have,
through long-term adaptation, adjusted to cope
with seasonal fluctuations of freshwater input,
caused by seasonality of flow from river basins.
One impact of dams is that they often reduce or
eliminate this seasonality, in that water is stored
at times of high flow, and released at times of
low flow.
Whereas dams are built to constrain fresh water
in rivers, barrages are built across estuaries to
withhold water on the ebb-tide for the purposes
7.4.1 Anthropogenic impacts on rates and styles
of sedimentation
Reduced freshwater flow will produce signific-
ant changes in the functioning of deltas and
estuaries. In the former, reduction in sediment
supply can lead to increased marine influence,
and thus increased erosion, whereas in the latter,
an estuary may experience changes in flow
structure, and ebb/flood dominance. One of the
main ways of affecting the amount of riverine
flow is by the construction of dams. Completion
of the Aswan High Dam in 1964, for example,
reduced the supply of sediment to the Nile delta
from 124 to 50 million tonnes a year (Carter
1988). The net result was a dramatic increase
in coastal erosion along parts of the delta edge
(see Case Study 7.2). In a similar way, closure
of the Akosombo Dam in 1961 completely
blocked sediment supply to the Volta delta in
Ghana (Ly 1980). Apart from initiating large
amounts of coastal recession, the salt wedges in
the delta channels moved further landwards,
altering local ecology. Furthermore, pollutant
flushing was reduced leading to increased resi-
