Abstract
Due to climatic constraints in dryland regions, wetlands usually occur at confluences of flow paths, whether from surface flow, inter-flow or at locations of groundwater discharge. Long-term landscape processes that shape valleys and focus the movement of water and sediment are accountable for providing a suitable template with which hydrology interacts to allow wetland formation. Current hydrogeomorphic classification systems do not address system-scale linkages of sediment and water transport across the landscape, and are therefore unable to contextualise long-term process dynamics. Misunderstanding long-term earth system processes can result in the application of inappropriate restoration strategies that isolate wetlands from longitudinal drivers of their formation. We propose a genetic classification system that focuses on the mode of wetland formation, and is based on the understanding that genetic processes impact on the outcome hydrology, sedimentology, geomorphology, ecosystem service provision and long-term dynamics of wetlands in drylands. The classification aims to impart understanding of dynamic processes of sediment transport through wetlands, such that restoration plans can be sensitive to long-term landscape processes. The classification system, derived from a combination of international literature and published South African case studies, has four wetland macrotypes based on sediment source (colluvial, alluvial, aeolian and geochemical). These are subdivided into 8 wetland types; hillslope seep, floodplain, valley-bottom, plain, blocked-valley, alluvial fan, aeolian depression and geochemical depression. The classification is based on landscape location, shape and the occurrence of geomorphic characteristics indicative of process.
Highlights
Another Wetland Classification System?The majority of wetlands in southern Africa are located along drainage lines due to widespread semi-arid conditions associated with the sub-tropical high pressure belt (Ellery et al, 2009)
We argue that a wetland classification system can be more than a system for inventory, and that it can be used as a tool to improve understanding of wetland processes and dynamics (Kondolf and Piégay, 2016)
We propose that a genetic geomorphic classification system in the context of dryland wetlands can provide more than a series of classes on which to base conservation efforts, but that this classification system will improve understanding of wetland geomorphic processes that govern the movement of water and all constituent matter through the ecosystem at varied spatial and temporal scales (Lisenby et al, 2019)
Summary
The majority of wetlands in southern Africa are located along drainage lines due to widespread semi-arid conditions associated with the sub-tropical high pressure belt (Ellery et al, 2009). A reed-dominated emergent wetland could occur in multiple landscape environments, such as in a tidal lagoon, on the bed of a non-perennial river or on the margins of a floodplain While this approach is useful for cataloguing purposes, there is no indication of the geomorphic or hydrological processes that are likely to drive long-term functioning. Ollis et al (2015) argue that the “functional unit,” which may be derived at level 5 by applying a secondary discriminator (saturation period) to the hydrogeomorphic type, defines the functioning of the aquatic ecosystem Functional units at this scale are not useful when planning restoration as this approach does not recognise that wetlands are part of an integrated drainage system with fluxes of sediment and water between functional units that are as important as the units themselves While wetland restoration planning should consider the restoration of habitat patches in order to conserve threatened species or plant communities, this should be done at a system scale to ensure system integrity
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