The Mgeni Estuary pre- and post Inanda Dam Estuarine dynamics.

Abstract

The funnel-shaped Mgeni Estuary, at the mouth of the fourth largest river along the KwaZulu- Natal coast, discharges into the Indian Ocean in the northern suburbs of Durban. This system is under considerable anthropogenic stress associated with modern development and industrialisation, both adjacent to the estuary and in the catchment area. The construction of Inanda Dam in 1989, immediately upstream of the estuary, is considered to have had an irreversible impact on the system and the estuary and is addressed in this study. A thorough analysis of available maps, aerial photographs and oblique images from 1860 to 2006, provided a record of the changes in the estuary morphology. The Pre-Inanda Dam period is characterised by the repeated re-establishment of a large central bar after major floods as evident prior to the 1987 flood. The post 1989 Inanda-Dam period, however, is characterised by the deposition of a series of side-attached bars and the development of extensive mudflats. Sediment distribution results show a definite decrease in grain size compared to the predominant gravel fraction in 1986 towards an estuary dominated by medium to fine sand-sized sediment. A reduction in sediment grain size is also complemented by an increase in carbonate concentration throughout the estuary. This indicates an increase in marine incursion with decreased fluvial flow suggesting a shift from a river dominated estuary to a marine dominated system since the completion of Inanda Dam. Linked to the finer grained sediment is a higher increased organic and heavy metal concentration. An analysis of the enrichment factor for 9 metals shows that the Mgeni Estuary is significantly contaminated, especially at the head of the estuary. This situation is exacerbated by decreased fluvial flow rates, an increase in the frequency of mouth closures and the reduction in flooding events effectively preventing the removal of contaminants out of the estuary.