Aquatic marginal wetlands cannot be considered
in isolation from their parent
water bodies.
A fringe wetland will merge with the open
water body which it fringes, and most
large wetlands incorporate stretches of open water from which they cannot
realistically be functionally separated. Any seasonally flooded wetland, for example,
will have channels into which water drains, channels which in managed wetlands
are straightened but otherwise equivalent to those which would occur naturally.
Saltmarshes and mangals (mangrove swamps) contain networks of drainage
channels, permanently flooded or into which sea will penetrate during every high
tide.
River floodplains may contain relatively
large, permanent water bodies such as
oxbow lakes, remnants of former river channels. These have no clear or permanent
inflow, but are replenished by groundwater percolation and by occasional
inundation during floods. Their equivalents in coastal wetlands are lagoons,
generally physically separated from the adjacent sea and virtually tideless, but with
some influx of sea water, particularly during spring tides.
Despite being formed or maintained without
the requirement for open water
bodies, mires will not exist independently of other aquatic systems. If a wetland has
a fluctuating topography, some parts will inevitably fall below the water table,
allowing open water bodies to form.
A major difference between aquatic marginal
and mire wetlands, in terms of their
relationships to other water bodies, is that, whereas aquatic marginal wetlands owe
their existence to rivers, lakes or the sea, mires, as reservoirs of water, actively
contribute to the formation of other aquatic systems. Even blanket bogs, maintained
solely by rainwater, will almost inevitably act as a source for rivers, and commonly
their surfaces are dotted with open pools. This distinction does, however, mask the
role of flood wetlands in regulating the flow of rivers. Such wetlands act as
reservoirs of excess flood water, much of which will gradually drain back into the
channel, dampening major fluctuations in discharge; isolated from its wetlands, a
river's discharge may fluctuate rapidly in response to precipitation and, during dry
periods, it may cease to flow altogether.
