BARRIER/ESTUARINE PROCESSES; BOT RIVER ESTUARY—AN INTERPRETATION OF AERIAL PHOTOGRAPHS

SUMMARY

Bot River estuary can be classified as an end-member stage of a bar-built estuary, occupying a drowned river valley. The modern barrier consists of fairly large back-barrier dunes, hummocky foredunes and a dissipative beach.

Aerial photographs were interpreted to assess whether the estuary has during modern times always had its present configuration, and to interpret the present physical processes based upon changing morphological features.

Lowering of sea-level following the Late Holocene high-water stand exposed a sandy strand plain between Kleinmond and Hawston. Wind action forced an eastward transport of these sands creating a large dune field between Sonesta and Hawston, and a deflation surface at Rooisand. In 1938 the Lamloch swamps were well vegetated but by 1981 large formerly vegetated areas had been converted to open water as a result of subsidence.

Long periods of mouth closure forced high water-levels in the estuary which flooded the deflated Rooisand area. Eventually estuarine waters made their way into the subsiding Lamloch swamp via two distinct pathways. Creation of the two channels reflects the natural adjustment of the water bodies to closure of the estuary mouth. Conceivably, barring man's intervention, the system could revert back to an estuary experiencing more regular marine interchange. That is, the estuary would consist of a brackish coastal lake connected by a sinuous estuary (Lamloch and Rooisand areas) to the sea at Kleinmond.

The modern barrier is prograding landward into the estuary due to sediment introduced during washovers. Barrier accretion is enhanced when the barrier fronting the estuary is artificially breached.