A Simplified Method to Assess the Impact of Sediment and Nutrient Inputs on River Water Quality in Two Regions of the Southern Coast of South Africa

Many rivers in the southern coastal region of the Western Cape Province, South Africa, are known to be in a poor state. Since the 1990s, the river water quality of this coastal region has been affected by increasing populations and by intensifying land use activities. Simplified risk assessment approaches are critical to identify in a timely manner areas where land use activities may impact water quality, particularly for regions with limited data. For this study, a simple assessment approach to estimate the impacts of land use activities on river water quality was improved by incorporating landscape potentials that take into account environmental factors. The methods were applied to two regions experiencing intensive land use along the southern coast. The findings indicate that the incorporation of the landscape potentials, (i) the landscape sediment generation potential and (ii) the diffuse nitrate potential, to estimate the impacts of sediment and nutrient inputs on river water quality need to be considered. Agricultural activities and informal settlements contribute to the increasing sediment and nutrient inputs of the river reaches. Areas with high proportions of river reaches at increasing pollution risk need to be managed on a large scale to ensure that all the potentially affected sub-catchments are included.