Floodplain Wetland
Floodplain Wetland
—a wetland area on the mostly flat or gently-sloping land adjacent to and formed by an alluvial river channel, under its present climate and sediment load, which is subject to periodic inundation by overtopping of the channel bank.
Floodplain wetlands, as the name implies, generally occur on a plain and are typically characterised by a suite of geomorphological features associated with river-derived depositional processes, including point bars, scroll bars, oxbow lakes and levees
Floodplain wetlands must be considered as wetland ecosystems that are distinct from but associated with the adjacent river channel itself, which must be classified as a ‘river’. Remember that some river channels, especially in the more arid parts of South Africa, are vegetated.
The definition of a floodplain wetland contains several key points that you should take note of. Firstly, a floodplain is a depositional surface formed by an alluvial river, not an erosional surface or a surface formed by other non-riverine processes that can deposit sediments (such as winddriven sands). Secondly, the floodplain is formed under the current climate and sediment load. Flat surfaces may be present along the margins of a river from previous eras of differing climate and/or sediment load, and these surfaces are called terraces. Terraces are generally not geomorphologically active, that is, they are not currently being built by river depositional processes. Finally, the floodplain is flooded (or inundated), on average, several times per year, during moderate peak flow events (such as a 1.5-year or 2-year flood). Terraces may be overtopped, but only by larger, less frequent floods (e.g. 50-year or 100-year events). The inner edge of the floodplain is called bankfull. The bankfull channel, or active channel, refers to the channel cross-section below the elevation of the floodplain.
Regular (or significant historical) water and sediment contributions from an associated river channel are what characterise the dynamic nature of a floodplain wetland. Another key characteristic of most floodplain wetlands is that they are generally located on a plain in terms of their landscape setting, although they can occur along the floor of a relatively wide valley with a low gradient (e.g. alongside the lower reaches of a Lower Foothill River).
Water and sediment enter floodplain wetlands mainly as overspill from a major river channel during flooding. Water movement through the wetland is predominantly horizontal and bidirectional (i.e. in and out of the wetland), in the form of diffuse surface or subsurface flow, although significant temporary containment of water may occur in floodplain depressions. Water generally exits a floodplain wetland as diffuse surface and/or subsurface flow (often returning to the river channel), but infiltration and evapotranspiration of water from a floodplain wetland can also be significant, particularly if there are a number of depressional areas within the wetland.
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