Marshland

I

Introduction

Marshland, treeless land in which the water table is at, above, or just below the surface of the ground; it is dominated by grasses, reeds, sedges, and reed mace. These plants typify emergent vegetation, which has its roots in soil covered or saturated with water and its leaves held above water. A different type of wetland, developing in flat, upland regions with high precipitation but not necessarily close to bodies of open water, is peatland.

II

Types of Marsh

Marshes may be freshwater or salt. Freshwater marshes develop along the shallow margins of lakes and slow-moving rivers, forming when ponds and lakes become filled with sediment. Salt marshes occur on coastal tidal flats. Inland salt marshes occupy the edges of saline lakes. The nature of a marsh—its plant composition, species richness, and productivity—is strongly influenced by its relationship to surrounding ecosystems. They affect the supply of nutrients, the movement of water, and the type and deposition of sediment.

Salt marshes are best developed on the Atlantic coasts of North America and Europe. In eastern North America the low marsh is dominated by a single species, salt-marsh cordgrass. The high marsh consists of a short cordgrass called hay, spike grass, and glasswort. Glasswort is the dominant plant of Pacific Coast salt marshes.

III

Water and Vegetation Patterns

In some marshes, such as the saw-grass wetlands of the Everglades or in salt marshes that are swept twice daily by tidal floods, water flows like a sheet across the surface, and the terrain is typically dominated by one or two species of emergent vegetation. In other marshes the water flows in channels rather than in sheets, flooding only at times of snowmelt and heavy precipitation and bringing in nutrients and sediment. Such irregular deposition of sediments provides variations in water depth, thus creating conditions favourable for a variety of wetland species. Deep marsh water is colonized by aquatic submerged plants (pond weeds) and floating plants (pond lilies). Shallower water supports reeds and wild rice. Very shallow water supports sedges, bulrushes, and reed mace.

As sediments and organic deposits raise the bottom of a marsh above the water table, aquatic vegetation is gradually replaced by shrubs and eventually by a terrestrial ecosystem of upland grasses or forest trees.

IV

Importance

Freshwater marshes provide nesting and wintering habitats for waterfowl and waders, muskrats, frogs, aquatic insects, and many other examples of freshwater life. Salt marshes are wintering grounds for snow geese and ducks, a nesting habitat for herons and rails, and a source of nutrients for estuary waters. Marshes are important in flood control, in sustaining high water tables, and as settling basins to reduce pollution downstream. Despite their great environmental value, marshes are continually being destroyed by drainage and filling. This process has been going on throughout recorded history: many famous cities have been at least partially constructed on land reclaimed from marshes, including Rome, Brussels, Tokyo, and St Petersburg. In the fens of East Anglia, in Great Britain, marshland has been drained in a continuous process over several centuries, mainly for agricultural use, but with socio-political consequences related to land ownership as well. A startling current example of the political importance of marshland is in southern Iraq, where the Iraqi government is draining the area around the Shatt Al Arab. This has been widely denounced as an attack on the habitat and culture of the people living there, aimed at reducing their political opposition to the Iraqi government.