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Controls on the formation of Wakkerstroom Vlei, Mpumalanga province, South Africa

The present study investigated controls on the formation of Wakkerstroom Vlei, an ∼1 000 ha unchannelled valley-bottom wetland on the South African Highveld. Along the uppermost and lowermost reaches of the wetland, where dolerite outcrops occur along the main valley, hydrogeomorphic features typical of geological control on wetland formation are present, including a meandering river channel, oxbow lakes and <3 m deep organic-poor alluvial fill. Along the main body of the wetland, however, floodplain features are absent, alluvial fill is up to 4.5 m deep and contains up to 2 m deep accumulations of peat. These characteristics deviate from the traditional model of dolerite control on wetland formation described by several other studies on Highveld wetland systems. The formation of Wakkerstroom Vlei has been controlled by a combination of geological, geomorphic and biotic factors, including the formation of a relatively wide, gently sloping valley along the middle reaches, creating accommodation for floodplain wetland formation, followed by impoundment of the main channel by laterally impinging alluvial fans, and the dense growth of robust vegetation, which has promoted flood-out of the main channel and peat formation. This knowledge has been important for identifying catchment and wetland land-use practices that threaten the longevity of Wakkerstroom Vlei.