SEDIMENTARY PROCESSES IN THE RIVER-DOMINATED MVOTI ESTUARY, SOUTH-AFRICA

The Mvoti River enters the Indian Ocean on the Natal coast of South Africa. In its lower reaches the river comprises a narrow, shallow channel that traverses a broad floodplain. At the coast, an outlet to the sea is maintained through a sandy barrier across an underlying rock outcrop. Seawater intrusion through the outlet is prevented by the elevation of the rock outcrop and hence reversing currents and tidal salinity variations typical of estuaries are not developed in the lower reaches of the river. Sedimentary deposits and processes in the back-barrier area are predominantly fluvial and the system is best classified as a river-dominated estuary: a lack of coastal progradation prevents its definition as a delta. The greatest geomorphological impacts in the channel and barrier are caused by extreme river floods. Sedimentary processes revealed by aerial photography and field measurements in the Mvoti show changes in response to discharge variation. Within the normal range of discharge variation the channel varies in dimension and morphology but no major long-term buildup of sediment is evident in the floodplain or channel. Minor floods deposit muddy overbank deposits. During such floods, the channel gradient is increased by barrier breaching and channel shortening, but wave-dominated barrier processes act to close the breach rapidly. Superimposed on these short-term changes is a cyclic pattern driven by major floods which cause lateral channel erosion and overbank deposition. Post-flood recovery involves progressive channel confinement and stabilisation as flow reverts to a single or braided channel and the floodplain is re-vegetated. After major floods the channel assumes a position in the centre of the floodplain but subsequently it shows a tendency to migrate to the edge of the floodplain and elongate its channel. Current incorrect perceptions of the river-mouth area as an estuary degraded by siltation are based on a comparison of present morphology with typical wave-dominated estuaries, and lack of recognition of the Mvoti as one of a suite of river-dominated estuaries. The study highlights the need for a suite of universally applicable models not only for estuaries, but for transitional fluvio-marine environments in general.