THE BATHYMETRY, ENVIRONMENTAL PARAMETERS AND SEDIMENTS OF THE BOT RIVER ESTUARY, S.W. CAPE PROVINCE

SUMMARY

The bathymetry of the Bot River estuary, water-level records from 1978 to 1983, wind records for a three-week period in August-September 1984, and investigations into the particle-size distribution, mineralogy and chemical composition of sediments in the estuary are reported and discussed.

The Bot River estuary is generally shallow, too shallow for the development of a halocline. The water-levels fluctuate between approximately 0,2 m and 2,7 m above mean sea-level. Water-flow through an overflow channel into the Lamloch swamps, during periods of high water-level, is sufficient to lower the water-level by 0,11 m per week, assuming no inflow. This is probably the main factor preventing the estuary from opening naturally to the sea. From August to October 1984 wind speeds exceeded 25 km h⁻¹ approximately 15–20% of the time, and blew from the north, west and east for approximately equal proportions of the time. The sediments in the estuary are mainly sands (51 %) or muds (33%). There is a marked lack of very fine sand in the estuary, and the mean particle size is medium sand. The sands consist mainly of quartz, except in the lower reaches near the sea where they contain a high proportion of shell fragments. The compositions of the muds in the central mud belt are remarkably homogeneous, and probably reflect thorough mixing of the finer particles by resuspension caused by wind-generated wave and current action on the sediments. The sediments in the estuary consist of quartz alone, or simple mixtures of quartz and carbonates (shell fragments) or quartz and clay minerals (with some organic matter). Only 4 out of 199 sediment samples show any mixing of carbonates with clay minerals.