Late Quaternary dynamics of a South African floodplain wetland and the implications for assessing recent human impacts

Knowledge of the long-term geomorphological dynamics of wetlands is limited, so currently there is an inadequate scientific basis for assessing anthropogenically induced changes and for developing conservation, remediation, and/or sustainable management guidelines for these fragile ecosystems. Along the upper Klip River, eastern South Africa, geomorphological and sedimentological investigations, geochronology, and remote sensing have been used to establish the late Quaternary dynamics of some internationally important floodplain wetlands, thus providing a reference condition against which to assess the extent of recent human impacts. Optically stimulated luminescence dating reveals that the wetlands have developed over at least the last 30 ky as a result of slow meander migration (< 0.2 m y ^{- 1}), irregular cutoff events, and infrequent avulsions (approximately one every 3-6 ky) that have occurred autogenically as a natural part of meander-belt development. Following European settlement in the Klip valley (late nineteenth century), however, modifications to local flora and fauna, as well as the initiation of local wetland drainage schemes, have had major impacts. In particular, proliferation of exotic willows and associated debris jams, and the artificial excavation of a 1.2-km-long channel section across the wetlands have initiated an ongoing avulsion that is characterised by failure (gradual abandonment) of the main channel and rapid incision of a headcutting channel. Compared to the pre-settlement condition, little change in lateral migration activity has occurred, but this avulsion provides a clear example of anthropogenically accelerated change, occurring only ~ 1 ky after the last natural avulsion and in a part of the wetlands where avulsions have not occurred previously. Subsequent human interventions have included installing weirs in an attempt to control the resulting erosion and promote reflooding, but ongoing maintenance has been required. In areas that were not glaciated during the Quaternary, many other floodplain wetlands may be of similar antiquity, but the Klip River illustrates their sensitivity to direct and indirect human impacts.