273
18.6.3 Near Coast Marshes
These are large tracts of FW marshes concentrated along the northern coast of the Gulf of
Mexico and along the South Atlantic Coast of the
USA. They have vegetation and ecological functions almost similar to those of the Prairie potholes in many ways.
18.6.4 The Everglades
The Everglades of South Florida is the largest
single marsh system in the USA. It had originally
occupied an area of almost 10,000 km
2 extending
in a strip of up to 65 km width and 170 km in
length from Lake Okeechobee to the brackish
marshes and mangrove swamps of the SW
coast of Florida (Kushlan 1991 ). The Everglades
remaining today are still immense, although
some areas have been drained for agriculture.
18.6.5 Great Lakes Marshes
They had been formed originally in the deltas of
the rivers which fl ow into the lakes and in protected shallow areas, often behind natural barrier beaches or levees thrown up by wave action
on the shore or behind ancient beach ridges
(Herdendorf 1987 ). The marshes occur along the
shores of lakes, especially along the Laurentian
Great Lakes of North America (Mitsch and
Bouchard 1998 ; Keough et al. 1999 ). Many of the
remaining Great Lakes marshes are today managed and protected from water-level changes by
man-made dykes since shorelines have been stabilised along the Great Lakes (Mitsch 1989 ).
18.6.6 Riverine Marshes
Riverine wetlands are common throughout the
world and the North American continent.
Wetlands are particularly extensive in the fl oodplain of the Mississippi River Valley and the
many smaller rivers which empty into the South
Atlantic. These wetlands are mostly forested.
However, marshes often border the forests or
occupy pockets within them, sometimes in abandoned ‘oxbows’. These same backwater marshes
are called ‘billabongs’ in Australia. These small
marshes often harbour much wildlife, particularly waterfowl. There are plenty of river-formed
oxbow wetlands in India and Bangladesh. They
have been dealt with in Chap. 20 .
18.6.7 Hydrology
The fl ooding regime or hydro-period of FW
marshes determines their ecological character as
with any other wetland. Some of the critical factors which determine the character of these wetlands are the presence of excess water and sources
other than direct precipitation. Excess water occurs
when precipitation exceeds evapotranspiration and
also when the watershed drains into the marsh
which is large enough to provide adequate run-off
or groundwater infl ow. In contrast, the water levels
in inland marshes are much controlled by the balance between precipitation and evapotranspiration. Many marshes such as wet meadows, sedge
meadows, vernal pools and even Prairie potholes
dry down seasonally. However, the plant species
found there refl ect the hydric conditions which
exist during most of the year. The seasonality of
these marshes is due to the fact they are fed primarily by run-off and precipitation. Further, some
marshes may also intercept groundwater supplies.
18.6.8 Climatic Variability
The wetland levels, precipitation, etc., are often
notoriously variable because of river and stream discharge, weather shifts from year to year, etc. Further,
the water regime of most inland marshes also varies
in a way which is predictable only in a statistical
sense (La Baugh 1986 ; La Baugh et al. 1998 ).
18.6.9 Biogeochemistry
The water and soil chemistry of FW marshes is
dominated by a combination of mineral rather
18.6 Freshwater Marshes: Inland Wetlands
18.6.3 Near Coast Marshes
These are large tracts of FW marshes concentrated along the northern coast of the Gulf of
Mexico and along the South Atlantic Coast of the
USA. They have vegetation and ecological functions almost similar to those of the Prairie potholes in many ways.
18.6.4 The Everglades
The Everglades of South Florida is the largest
single marsh system in the USA. It had originally
occupied an area of almost 10,000 km
2 extending
in a strip of up to 65 km width and 170 km in
length from Lake Okeechobee to the brackish
marshes and mangrove swamps of the SW
coast of Florida (Kushlan 1991 ). The Everglades
remaining today are still immense, although
some areas have been drained for agriculture.
18.6.5 Great Lakes Marshes
They had been formed originally in the deltas of
the rivers which fl ow into the lakes and in protected shallow areas, often behind natural barrier beaches or levees thrown up by wave action
on the shore or behind ancient beach ridges
(Herdendorf 1987 ). The marshes occur along the
shores of lakes, especially along the Laurentian
Great Lakes of North America (Mitsch and
Bouchard 1998 ; Keough et al. 1999 ). Many of the
remaining Great Lakes marshes are today managed and protected from water-level changes by
man-made dykes since shorelines have been stabilised along the Great Lakes (Mitsch 1989 ).
18.6.6 Riverine Marshes
Riverine wetlands are common throughout the
world and the North American continent.
Wetlands are particularly extensive in the fl oodplain of the Mississippi River Valley and the
many smaller rivers which empty into the South
Atlantic. These wetlands are mostly forested.
However, marshes often border the forests or
occupy pockets within them, sometimes in abandoned ‘oxbows’. These same backwater marshes
are called ‘billabongs’ in Australia. These small
marshes often harbour much wildlife, particularly waterfowl. There are plenty of river-formed
oxbow wetlands in India and Bangladesh. They
have been dealt with in Chap. 20 .
18.6.7 Hydrology
The fl ooding regime or hydro-period of FW
marshes determines their ecological character as
with any other wetland. Some of the critical factors which determine the character of these wetlands are the presence of excess water and sources
other than direct precipitation. Excess water occurs
when precipitation exceeds evapotranspiration and
also when the watershed drains into the marsh
which is large enough to provide adequate run-off
or groundwater infl ow. In contrast, the water levels
in inland marshes are much controlled by the balance between precipitation and evapotranspiration. Many marshes such as wet meadows, sedge
meadows, vernal pools and even Prairie potholes
dry down seasonally. However, the plant species
found there refl ect the hydric conditions which
exist during most of the year. The seasonality of
these marshes is due to the fact they are fed primarily by run-off and precipitation. Further, some
marshes may also intercept groundwater supplies.
18.6.8 Climatic Variability
The wetland levels, precipitation, etc., are often
notoriously variable because of river and stream discharge, weather shifts from year to year, etc. Further,
the water regime of most inland marshes also varies
in a way which is predictable only in a statistical
sense (La Baugh 1986 ; La Baugh et al. 1998 ).
18.6.9 Biogeochemistry
The water and soil chemistry of FW marshes is
dominated by a combination of mineral rather
18.6 Freshwater Marshes: Inland Wetlands