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Newsletter and Technical Publications
Lakes and Reservoirs vol. 2
The Watershed: Water from the
Mountains into the Sea
Wetlands and Flooded Areas: Water on Perpetually-Saturated Soil
Another important component of the hydrologic cycle is wetlands, which
comprise the land areas that are partially or permanently saturated with water
during some or all of the annual cycle. The international Ramsar Convention
defines wetlands broadly as “areas of marsh, fen, peatland or water; whether
natural or artificial, permanent or temporary, with water that is static or
flowing, fresh,
brackish or salt, including areas of marine waters, the depth of
which at low tide does not exceed six metres ” and may include “riparian and
coastal zones adjacent to the wetlands or island or bodies of marine water
deeper than six metres at low tide lying within ”. For the purposes of this
booklet, however, wetlands comprise marshes, swamps and other heavily vegetated
areas (bogs, peatlands, fens, etc. ) saturated with water.
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| Photo 35: Wetland in Lake Biwa, Japan |
A major wetland characteristic is the continuing presence of water either at
the land surface or within the root zone of plants. Wetlands (Photo 35), are
found throughout the world, ranging in size from a few hectares to thousands of
square kilometres. They are often located along the edges of lake basins, on
river floodplains and on rivermouth deltas. It is estimated that approximately
6% of the Earth’s surface is covered with wetlands, equivalent to about 8.6
million square kilometres. Wetland vegetation can be quite varied and dense and
include substantial accumulations of dead and partially decayed trees and
plants. Wetlands also hold a large variety of plants and animals (Photos 36, 37,
38 and 39). Some types of wetlands are specific to certain regions, an example
being the mangrove swamps associated with coastal areas (Photo 40). The latter
are especially important as a buffer zone between the land and coastal waters.
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| Photos 36, 37, 38, and 39: Biodiversity in
wetlands. |
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The most important factor controlling the structure and function of a wetland
is its hydrology. Wetlands are highly dependent on the upstream conditions in
their watershed, in regard to both water quantity and quality. The degree of
soil saturation of a wetland will obviously depend on the magnitude and
consistency of its freshwater inflows. As with upstream rivers and lakes, the
quality of the inflowing water to a wetland is a function of the point and
non-point pollutant sources in the watershed, and water pollution can readily
modify the natural
flora
and fauna
of a wetland.
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| Photo 40: view of the mouths of the Irrawaddy River, Myanmar, taken from
a satellite. The pinkish colour shows mangrove forest. |
Compared to rivers, lakes and groundwater, the range of direct human water
uses associated with wetlands is limited. Nevertheless, wetlands are a vital
life support system for many biological communities. They also provide a range
of ecological and hydrological functions for human benefit, as well as goods and
services for human health and well-being. In addition to providing fish,
shellfish and other food resources, wetlands also perform ecosystem services of
direct economic benefit to humans, including groundwater recharge and discharge,
flood storage, dissipation of natural erosion forces, sediment and nutrient
retention and removal, wildlife and fishery habitat, and water purification. In
fact, some of the ecosystem services provided by wetlands are not found
elsewhere.
Among the ecosystem services, flood control and water purification warrant
further mention. In regard to flood control, wetlands act as temporary storage
systems for water discharges from flood events. Wetland vegetation also retards
surface water flows, thereby hindering the downstream passage of flood waters.
This property can both reduce flood flows and peaks, and increase the duration
of reduced floodwater flows. In regard to water quality, wetlands have a
significant capacity to maintain and/or purify water passing through them.
Virtually all water- quality parameters can be altered by the passage of water
through a wetland; hence, their characterization by some as the “kidneys ” of
the landscape. Improved water quality can result from sediment deposition, as
suspended sediments and sediment-associated pollutants drop out of the water
column with reduced stream velocities in a wetland. A variety of biological and
chemical reactions in wetlands, as well as dense vegetation, can also transform
and/or remove certain chemicals from the water. |
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| Photo 41: Artificial wetland constructed to treat organic liquid waste,
Kenya. |
Because of their ability to remove or minimize pollutants, wetlands have been
used to purify domestic and industrial wastewater, either as a primary treatment
for untreated waters or as a supplemental “polishing ” treatment for partially
treated wastewaters. Both natural and artificially constructed wetlands (Photo
41), have been used for this purpose, with the primary constraint being the need
to insure that a wetland’s water-purification capabilities are not overloaded.
The latter consideration includes the types and quantities of water pollutants,
as well as the timing of their introduction into the wetland system.
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