Late Holocene development of a major fluvial discontinuity in floodplain wetlands of the Blood River, eastern South Africa

In dryland settings, most floodplain wetlands form in low gradient, low energy environments that are characterised by strong interactions between flow, sediment and biota. Some floodplain wetlands are only partly channelled or largely unchannelled, and represent major discontinuities in drainage networks, fundamentally influencing downvalley water and sediment transfer. In the > 15 km2 Blood River floodplain wetlands, located in subhumid to semiarid eastern South Africa, field investigations, aerial photographs, and optically stimulated luminescence (OSL) ages provide evidence for development of a major discontinuity during the very late Holocene. Between ~ 800 and 100 years ago, the wetlands were characterised by a through-going, meandering channel set within a floodplain up to 2.5 km wide. A sinuous channel remains in the lower part of the wetlands but during the last ~ 100 years major morphological and sedimentary changes have occurred upvalley. The former through-going, meandering channel has been replaced by a straighter channel that decreases in size downstream and terminates in a ‘floodout’, characterised here by an unchannelled reedbed. Small tributaries supply water and limited sediment to this floodout and another floodout located farther downvalley. Organo-clastic sediments > 3 m thick have accumulated in the floodouts as broad lobes, in places burying the former meander-belt sediments. On the steepened, downvalley sides of these lobes, small headcutting channels convey water that filters through the reedbeds. If headcutting through the lobes continues, a through-going channel may re-establish upvalley, possibly eventually linking with the sinuous but now moribund channel in the lower part of the wetlands. Along the Blood River, the initial cause(s) of the sequence of changes is not known, but these channel–floodplain adjustments are partially analogous to the system-scale, autogenic morphological and sedimentary dynamics of those dryland fluvial systems that are also characterised by a combination of channelled and unchannelled landforms (e.g. discontinuous ephemeral streams, erosion cells). Knowledge of these historical changes can provide scientific underpinning for present-day management of wetlands, including assessments of ecosystem service provision and the suitability of measures to control erosion.