Geoscience Reference
In-Depth Information
largely common. Susceptibility to changes in these
issues makes both systems highly vulnerable,
both in the long and short term. The last section
discussed management and the prevention of
some elements of change. Remaining problems
include: contaminant management and the legacy
of stored contaminants in many historically
industrialized estuaries; loss of habitat, largely
now the result of erosion and land claim for
development; freshwater management due to the
growing conflict between the need for increased
water resources for a growing population, and the
need to maintain freshwater flow into brackish
systems; and sea-level rise.
Some elements of change, such as erosional
and depositional cycles in marshes and in delta
channels, may show a cyclicity (see e.g. Case
Study 7.1). There is an increasing element of
directional change, however, which could see
these systems under increasing threat in the
future. The major decision for managers is to
recognize the point at which management has
to give up trying to maintain a
status quo
, and
allow change to happen. It is a great mistake for
contemporary scientists and morphologists to
regard all of our current environments as being
at the end of their evolutionary cycle. It is much
more likely that much of the current change is
part of a process of evolution and must, there-
fore, be allowed to continue for the sake of the
future stability of the environment. A key factor
to determine, however, is to what extent will
human activity accelerate or alter the rate and
direction of change. This final section will dis-
cuss some of these key issues.
Chapter 1). This is exacerbated for estuaries
and delta channels, which are intensively used
for development and, because of historic trends
of sea-level rise and of coastal erosion and flood-
ing, have seen increased use of coastal defence
measures to protect human interests. Ironic-
ally, this policy now threatens to increase the
vulnerability of habitats to loss through coastal
squeeze. In the UK there is not one estuary that
will not be vulnerable to coastal squeeze under
sea-level rise. This is because they either have
extensive defences or they are rock-bound (ria)
type systems. This has major implications for
intertidal mudflats and salt marshes. Similarly,
the coastlines of many deltas, and the margins
of their major distributary channels, are often
heavily defended and are thus also susceptible
to squeeze.
Impacts of sea-level rise can be seen over a
range of scales. On a large scale, the increased
loss of fringing salt marshes and greater pene-
tration of the salt wedge further upstream are
all ways of affecting the ecological and mor-
phological structure of the system as estuaries
widen and deepen. It is also feasible that the
increasing volume of sea water entering on the
flood tide may alter flood/ebb dynamics and
affect sediment depositional patterns. Further-
more, increased volumes of sea water within
estuaries may impede freshwater flow for longer
periods, leading to the possibility that in some
large systems, where the volume of sea water
is great, there could be an increased risk of
riverine flooding further into the catchment
(Bird 1993). Other, more subtle changes can be
equally important. Sea-level rise has caused
salinity increases in some estuaries, which have
led to faunal and floral changes. In Louisiana,
for example, salt marshes have replaced marginal
reed swamps as water salinity and frequency
of inundation have increased (de Sylva 1986),
and in the coastal lagoons of the Nile delta, such
as Lake Burullus (see Case Study 7.2), increas-
ing salinity of surface and groundwater has
affected fish populations (Bird 1993). Similar
salinity increases also occur in coastal ground-
water aquifers, thus posing an increased threat
to drinking water supplies.
7.6.1 Impacts of climate change and sea-level rise
There is some irony in the fact that estuaries
represent the drowned mouths of river valleys,
as this immediately ties their origin into sea-level
rise, yet today there is concern about the threat
of this very same process as an agent of change.
Sea-level rise, however, represents one of the
greatest issues in relation to contemporary coastal
systems. Any area that experiences tides will
experience changes caused by deepening sea
water and increased tidal penetration inland (see
