The Breede River estuary (Cape Province, South Africa): A historical perspective on hydrology, geomorphology, and sedimentology

A hitherto unpublished historical dataset of the Breede River estuary, Western Cape, South Africa, that was collected in the summer season of 1983 is presented. Bathymetric, physiographic, and sedimentological data were collected at 1–1.5-km intervals between the mouth and the Malgas ferry crossing at km 35. The remaining estuary up to km 52 was not surveyed. Sedimentologically, the estuary could be divided into a lower marine sand reach (mouth to km 5), a mixed sand/mud reach (km 5–18.5), and a fluvial sand reach upstream of km 18.5. Hydrological data were collected at three midstream anchor stations (mouth, km 14.5, and km 24) which were occupied for complete tidal cycles. Five parameters were recorded: tidal elevation, current velocity, salinity, temperature, and light transmittance. Pronounced velocity asymmetries of the tides were revealed by phase delays between the times of low (high) water and corresponding slack water (turn of the tide), as well as considerable up-estuary delays in the occurrence of high and low tides. The mode of tidal wave propagation was synchronic (constant height) up to a distance of ~23 km at spring high tide and ~13 km at neap high tide, from where it proceeded in hyposynchronic mode (progressive decrease in height). Peak surface velocities reached 1.5 m/s at Station 1 (mouth), 0.6 m/s at Station 2 (km 14), and 0.45 m/s at Station 3 (km 24). The marine sand reach and parts of the mixed sand/mud and fluvial sand reaches were distinctly flood dominated as revealed by the orientation of bedforms. Salt water intrusion reached up to km 25, where river background levels were reached. Suspended sediment concentrations (turbidity) varied from 55–85 mg/l at the mouth, 65–200 mg/l in the mixed sand/mud reach, and 55–85 mg/l in the fluvial sand reach. At the time of observation, the Breede River estuary was in a well-mixed hydrological state. The fluvial sand reach displayed numerous, up to 18-m-deep scour pools. Grain-size distributions revealed distinct differences between the individual estuary sections, the sand/mud, and marine sand reaches, in particular, being characterized by up to four, mostly mixed hydraulic populations. Offshore seismic profiles suggest that the Duiwenhoks River, located 14 km to the east of the Breede mouth, was a tributary of the latter during Pleistocene sea-level lowstands. From a global perspective, the Breede River estuary conforms physically to typical small estuaries that are only marginally impacted by human interferences.

Senckenberg Forschungsinstitut und Naturmuseum Frankfurt (3507)