Evaluation of river mouth dynamics along the Eastern Cape coastline, South Africa

Abstract

Small rivers that flow into the sea often terminate in estuaries or lagoons that may be separated from the sea by a sandy beach barrier. As a result of variations in barrier width and river discharge, these river mouths can be variously open, closed or partly open at different times. This behaviour reflects the interplay between the processes and properties of river systems (discharge, sediment supply, channel width, water velocity) and beach systems (beach width and height, grain size, wave regime, longshore processes). This study examines the dynamic behaviour of 32 river mouths located along 238 km of the coastline of Eastern Cape Province, South Africa, between 2000 and 2021 using Google Earth imagery. At each available time snapshot, individual river mouths were classified along a continuum as open, partly open or closed. Results show that nine river mouths were permanently open whereas 22 varied between the three states. Only one river mouth was partly open and none was closed for the entire period. Some spatial and temporal patterns were also identified. Commonly, adjacent river mouths may show the same patterns of opening/closing, which may reflect regional climate forcing. Fewer river mouths are open during winter/autumn compared to summer, likely reflecting rainfall seasonality. Thus, regional climate is considered to be the major control on river mouth dynamics, likely in combination with human activities that impact on river discharge, although the role of coastal sediment dynamics is significantly less well understood. The interplay between these different forcing factors requires further investigation.

Keywords:

• catchment land use
• coastal processes
• estuaries
• lagoons
• rainfall seasonality

Acknowledgements

This paper is based on the first author’s BSc Honours project in 2021, supervised by the second author. We thank three anonymous reviewers for their comments on this paper.

SUPPLEMENTAL DATA

Supplemental data for this article can be accessed at: https://doi.org/10.1080/0035919X.2023.2190179.