Geoscience Reference
In-Depth Information
Fig. 7.10
Erosion of salt marshes caused by the stripping of protective vegetation. Note how the salt marsh vegetation has been peeled
back from the sediment surface, exposing the underlying sediments to wave attack (Silverdale salt marsh, Morecambe Bay, UK).
waves have stripped the vegetation surface from
the marsh, leading to increased lateral marsh
retreat (Pringle 1995).
whereas port development or coastal defences
can alter the importance of marine inputs.
A reduced sediment supply can also be caused
by variations in the current activity which previ-
ously delivered that sediment to its site of deposi-
tion. For example, in a delta, a main channel may
deliver its load of sediment to one part of the
delta front. Were the bulk of that water supply
to switch, and start to flow to another part of the
delta, the original section of delta front would
lose its sediment supply and could cease to func-
tion as an active part of the delta, potentially
becoming erosional. The diverted sediment supply
could lead to the formation of new active areas
of delta accretion. Over time, therefore, a delta
may reveal a series of accretional phases, each
being marked by a delta lobe. A good example
of this is the Mississippi, a large delta system
with a series of lobes dating from
c
. 7500 yr BP
(Fig. 7.11). In intertidal parts of estuaries, a
similar process of current change as a result of
7.3.2 Effects of changes in sediment supply
Although marsh erosion is significant, perhaps
the greatest threat to deltas and estuaries is
to reduce the volume of sediment available for
deposition. The relationship between a prograd-
ing delta front and an eroding one, or an advan-
cing salt marsh as opposed to a retreating one,
is closely linked to sediment supply. Figure 7.3
shows the complexity of sources and inputs to
an estuary or delta region, each varying in import-
ance in terms of what they contribute to the
sediment budget. Some sources are more easily
varied than others. For example, hinterland
activity can change the importance of urban or
agricultural runoff, which will lead to increases
or decreases in sediment delivery to the estuary,
