Download Freshwater deltas: St Clair Delta

Freshwater deltas: St Clair Delta
Ewa Szalinska
Institute of Water Supply and Environmental Protection, Cracow University of Technology,
31-155 Krakow, Poland
1. Definition and classification of freshwater wetlands
There are more than 50 definitions of wetlands in use throughout the world. Among these the
broadest, and therefore the one most widely used at the international scale, is provided by the
Ramsar Convention on Wetlands of International Importance: “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 water the depth of which at low tide
does not exceed six meters” (UNESCO, 1994). Except for Antarctica, wetlands can be found on
nearly all continents. Wetlands vary widely because of regional and local differences in soils, topography, climate, hydrology, water chemistry, vegetation and other factors, including human
disturbance (USEPA, 1990).
Two fundamental designs for wetland classification exist: horizontal and hierarchical. The
horizontal classification system divides habitats into a series of classes or types and tends to be
very general. These classifications included terms such as: marsh or swamp, which are familiar
to the public and useful for describing wetlands to a non-technical audience. Unfortunately,
many of these common terms lack universally accepted definitions. The hierarchical system provides a matrix for separating wetlands into a multitude of types depending on their features, i.e.
vegetation, hydrology, origin, etc. Hierarchical systems allow for more descriptive characterization of wetlands (Tiner, 1999). The Ramsar Convention identifies three general categories of
wetlands: coastal, inland (freshwater) and human-made.
Inland (freshwater) wetlands are most common on floodplains along rivers and streams
(riparian wetlands), in isolated depressions surrounded by dry land (for example, playas, basins
and “potholes”), along the margins of lakes and ponds, and in other low-lying areas where the
groundwater intercepts the soil surface or where precipitation saturates the soil leading to vernal
pools and bogs. Many of these wetlands are seasonal, i.e. they are dry one or more seasons every
year and may be wet only periodically. The quantity of water present, and the timing of its
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presence, partially determine the functions of a wetland and its role in the environment. The
most common types of freshwater wetlands include marshes, swamps, bogs and fens.
2. Similarities and differences between inland and coastal wetlands
The most important difference between inland and coastal wetlands, apart from water salinity, is
the flood pattern. The water level within inland wetlands remains relatively steady, except for
seasonal changes. The coastal salt marsh typically floods twice a day, while also revolving
around a monthly pattern of spring and neap tides. Both types of wetlands are subjected to succession, as the natural process of development is led by successive development of various plant
communities which alter the environment. This process can also be provoked by external factors
such as climate change. For coastal wetlands, mangrove expansion is the factor promoting consolidation and extension of the land (Dugan, 1993).
At geological timescales, inland wetlands are usually permanently evolving into new
shapes or structures (rivers, valleys, floodplains, deltas), which retain their specific features for a
very short time. The same applies to coastlines, which are rarely static for significant periods of
time. Furthermore, major fluctuations of sea level have been recorded during the last two million
years. Unless coastal wetland ecosystems can adjust to the rate of sea level changes, such as
through increased sedimentation, their water cycles will be subjected to major alterations.
Both inland and coastal wetlands are highly sensitive to climate change. Arid and
semi-arid areas are especially vulnerable to changes in precipitation; a decline in precipitation
can dramatically affect wetland areas. Increases in temperature in tundra and polar areas are anticipated to result in the melting of permafrost. This, in turn, will cause a reduction in the areal
extent and depth of tundra wetland. Subsequent changes in tundra wetland ecosystems are projected to cause the migration of vegetation zones northward (Mitsch and Gosselink, 2007).
3. Functions and values of inland wetlands
Wetlands store and slowly release surface water, rain, snowmelt, groundwater and flood waters.
Trees and other wetland vegetation also impede the movement of flood waters and distribute
them more slowly over floodplains. This combined water storage and slowing action lowers
flood heights and reduces erosion downstream and on adjacent lands. It also helps to reduce
floods and it prevents waterlogging of agricultural land. Some wetlands maintain stream flow
during dry periods, while others replenish groundwater.
Wetlands help to improve water quality, including that of drinking water, by intercepting
surface runoff and removing or retaining inorganic nutrients, processing organic wastes, and
reducing loads of suspended sediments before they reach open water. Wetlands also diminish
environmental problems, such as algal blooms, dead zones, and fish kills, that are generally
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associated with excess nutrient loadings. However, this function of wetlands can be limited. Too
much surface runoff carrying sediments, nutrients and other pollutants can degrade wetlands.
Wetlands can also be thought of as “biological supermarkets”, as they produce great quantities of food that attracts many animal species. The complex, dynamic feeding relationships
among the organisms inhabiting wetland environments are referred to as food webs. The combination of shallow water, high levels of inorganic nutrients, and high rates of primary productivity in many wetlands is ideal for the development of organisms that form the base of the food
web (Dugan, 1993). The economic, recreational, and educational benefits of wetlands are extremely important.
Until recently, there were several incentives for wetland drainage, particularly for industrial and agricultural purposes. Dredging, construction, creation of levees and dikes, as well as
chemical contamination, have led to a dramatic decrease in the number of wetlands. This loss has
already resulted in increased flooding and drainage problems together with impacts on native
wildlife populations. In addition to the human factors contributing to the loss of wetlands, environmental events also add to their decline, particularly factors such as increasing temperature, which
may be the result of global warming. Aside from rising sea levels which take over wetlands,
droughts, hurricanes and general erosion are also partly responsible for the decrease in wetlands.
While the causes for worldwide wetland loss vary, it is clear that human activity has had the
greatest impact. It should be noted that the loss of wetlands always triggers a chain reaction because damage or loss of one aspect of a wetland does not only have an impact on that one particular issue. On the contrary, it has more far-reaching consequences that often have great and even
devastating effects on human and animal populations as well as on the ecosystem as a whole.
4. St. Clair wetlands
The St. Clair wetlands are located in the St. Clair River delta and the watershed of Lake St. Clair
(Great Lakes, North America). Lake St. Clair is part of the Huron-Erie Corridor connecting the
Upper and Lower Great Lakes. It was formed during the retreat of the last glacier about 12,000
years ago. The last geological modification and shift in drainage took place about 5,500 years
ago. Rising post-glacial forms closed the northeast outlet from the upper lakes and caused the
water to flow southwards through the St. Clair River, Lake St. Clair and the Detroit River
(USACE, 2005).
Within the outlet of the St. Clair River, a unique freshwater delta has been created (Figure 1).
The delta consists of seven active deep channels (with an average water depth of 11 m) which
enter a lake with a mean depth of 3 m, with much shallower water in front of the delta region. The
channels are stable and are actively cutting into the sediments of the lake creating both
sub-aqueous and sub-aerial levee deposits with crevasses. The inter-distributary bays are being
filled with sandy deposits that have been redistributed by waves from the crevasse deposits.
At the erosional front of each distributary, a narrow erosional notch, or “leading channel” is
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Figure 1 Location of the St. Clair Delta (After Thomas et al. 2006)
being formed which appears to control the direction of the lake-ward erosion of each deep
channel. The emplacement of the delta body in the shallow receiving water body has been
termed a “burrowing” delta formation and is the main mechanism controlling this continually
formed sedimentary feature (Thomas et al., 2006).
Until about 1800, the land along the St. Clair River and Lake St. Clair was covered mostly
by wooded swamps. The passing of the U.S. Swamp Land Act and the Canadian Drainage Act in
the nineteenth century stimulated draining and conversion of this area into agricultural land.
Nowadays, the only remaining wetlands are located in the St. Clair Delta and the Thames River
plains (Chatham Flats) (USACE, 2005).
The Chatham Flats is mainly a marsh habitat with remnant patches of the tallgrass prairie
that used to be part of a larger prairie ecosystem. The water levels within this system are artificially controlled using impoundment techniques. A system of pumps and dykes mimics the
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Figure 2 Heavy metal and organic contamination in the St. Clair Delta sediments. LEL – Lowest
Effect Level (After GLIER, 2005)
natural rise and fall of water levels that triggers the diversity of plant growth. However, the St.
Clair Delta is not protected as a wetland system. The Canadian controlled area of the St. Clair Delta
belongs to the Walpole Island First Nations people (who rely on the landscape for food, water and
ceremonial traditions), while the USA portion is used as a residential and State game area.
The Walpole Island First Nations part of the Delta contains 6,900 ha of relatively undisturbed wetlands, as well as 12 per cent of Canada’s wildlife species at risk, including six plant
species found nowhere else in Canada. The delta provides an important breeding and migration
habitat for grassland birds, waterfowl, marsh birds and other migratory birds. The delta also
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provides habitat for a diverse array of mammals, reptiles, amphibians and rare invertebrates, as
well as plants.
The St. Clair wetlands are subject to many natural processes and stress factors. The most
important are direct wetland loss and degradation, invasive plant and animal species, and risks
from chemical and fuel spills. The sediment sampling programme on the Walpole Delta performed in 2005 (GLIER 2005) showed concentrations of pp’-DDT, HCB and Hg concentrations
exceeding sediment quality objectives (Figure 2). Over the last 150 years, the St. Clair wetlands
have undergone a dramatic transformation due to the anthropogenic impact. This transformation
has resulted in drained wetlands, loss of tallgrass prairie, fragmented forest habitats, increased
sedimentation, excess nutrient loading, and dredged aquatic habitats. Despite these changes, the
region remains of the most important inland wetlands in the Great Lakes region and also around
the world.
5. References
Dugan, E. (Ed.) 1993 Wetlands in Danger. A World Conservation Atlas. Oxford University Press,
New York.
GLIER 2005. Benthos and chemistry studies on the Detroit and St. Clair Rivers. Report Prepared
by the Great Lakes Institute for Environmental Research & Department of Biological
Sciences, University of Windsor for the Great Lakes Sustainability Fund, Windsor, Canada.
Mitsch, W.J., and Gosselink, J.G. 2007 Wetlands. John Wiley & Sons, Inc., New York.
Thomas, R.L., Christensen, M.D., Szalinska, E., and Scarlat, M. 2006. Formation of the St. Clair
River Delta in the Laurentian Great Lakes system. Journal of Great Lakes Research 32,
738748.
Tiner, R. 1999. Wetland Indicators. A Guide to Wetland Identification, Delineation, Classification
and Mapping. Lewis Publishers. Boca Raton FL.
USACE, 2005. St. Clair River and Lake St. Clair Comprehensive Management Plan, U.S. Army
Corps of Engineers, Detroit District (http://www.glc.org/stclair/)
UNESCO 1994 Convention on Wetlands of International Importance especially as Waterfowl
Habitat. United Nations Educational, Scientific and Cultural Organization, Paris,
USEPA 1990 Water quality standards for wetlands: national guidance, U.S. Environmental
Protection Agency, Office of Water Regulations and Standards, Office of Wetlands
Protection. Washington, DC (free copy available from: http://www.epa.gov/nscep/)