WHAT IS THE HISTORICAL RANGE OF FLOODPLAIN WETLAND VARIABILITY on THE SOUTH AFRICAN HIGHVELD?

On the South African Highveld, extensive floodplain wetlands occur in sandstone reaches between outcrops of dolerite that are relatively more resistant to erosion. Because the wetlands are important sites for migratory bird habitat, regional water resources, and ecotourism and geoheritage, many have received national and international recognition as critical sites for protection and special management. While management and restoration efforts are underway, there has yet been little scientific input on the historical range of geomorphic evolution and stability of the wetlands. Herein, we define the temporal range of variability and longevity of floodplain wetlands in the South African Highveld by using OSL techniques to date floodplain features on three rivers in different stages of wetland evolution.

We find that on the Klip River, which has a functioning wetland, the floodplains have been active over the last ~100 ka, at minimum. On the Schoonspruit, the wetland is desiccated. The wetland was active from ~1 Ma until at least 16.1 ± 1.5 ka, after which channel-floodplain decoupling occurred resulting in wetland abandonment. On the Mooi River, the wetland is intact but is in a transitional phase. Our initial work suggests that meander migration has occurred at a rate of ~0.45 m/yr in the floodplain wetland since 0.9 ± 0.08 ka. In the reach downstream of the wetland, the channel has incised through a dolerite sill into the underlying less resistant sandstone. The upstream end of the sill outcrops in the channel bed, marked by a large knickpoint in the long profile. Our findings suggest that following complete incision through the dolerite sill and exhumation by the channel of the underlying sandstone, rapid floodplain abandonment will occur.

While human impacts on the wetlands are not entirely negligible, our findings suggest that the long term stability of the wetlands is structurally controlled. Geomorphic wetland evolution in the region is dependent on changes in the local base level that occur in response to downcutting through heterogeneous, variably resistant lithologies. By defining the historical range of variability of the wetlands, we can inform on what elements of the wetlands are appropriate targets for management and restoration and on the temporal boundaries of what is achievable by those efforts.