The origin and dynamics of Wakkerstroom Vlei, Mpumalanga Province, South Africa.

Abstract

The formation and common occurrence of riparian wetlands within the semi-arid Highveld interior of South Africa, a landscape setting undergoing extensive long-term fluvial incision, is an enigma and the underlying controls on the formation and hydrogeomorphological dynamics of these wetlands has not been widely investigated. Wakkerstroom Vlei is one such enigma in that it is a large (~ 1000 ha) Highveld system comprising extensive reaches of unchanneled valley-bottom wetland with considerable (up to 2 m deep) peat deposits. Accommodation space for wetland formation is thought to be controlled by the superimposition of the main (Wakkerstroom/Thaka) river upon an erosion-resistant Karoo dolerite sill at the toe of the system, which forms a stable local base-level along the rivers course. As a result, the river has carved broad (up to 1300 m), gently sloping (average slope ~ 0.17 %) valleys along softer shale valley reaches upstream of the dolerite barrier. Examination of the valley fill along these valley-bottom wetland reaches, together with analysis of historic aerial photography, reveals that continuous tracts of meandering river and floodplain wetlands formerly existed, and that the wetland experienced an abrupt shift to valley-bottom wetland conditions where surface flow of water is diffusive. Following the creation of accommodation space along the main river valley, lateral tributary streams began to deposit substantial amounts of course sediment into the main valley via alluvial fans. Several of these fans have coalesced to form multiple coalescing alluvial fan complexes that historically were able to extend far across the floodplain from either side of the valley, resulting in main river valley impoundment. This has promoted flood-out formation, along the main valley which, together with the denser growth of vegetation across the floodplain, has created conditions suitable for organic sedimentation and peat accumulation. The formation and evolution of Wakkerstroom Vlei has thus been controlled by the complex interaction between geological, geomorphological and biotic processes. Understanding the role of these factors in shaping both the short- and long-term hydro-geomorphic dynamics of the system is essential in implementing effective management and conservation strategies both within Wakkerstroom Vlei and other large valley-bottom wetlands within the South African Highveld interior.