LAUSR

Small mountainous coastal basins display steeper gradients, suggesting intensified river transport. However, there is little information about downstream trends in rivers with low discharge and material fluxes across coastal plains and its influence on land‐sea material transport. This study examined 2 years of river discharge, suspended sediment and nutrient concentrations, fluxes and yields from upstream to the upper estuary of a typical South‐east Atlantic Basin. This study provides data about the material retention capacity of the coastal plain. The results indicated that the coastal plain did not affect nutrient concentrations, but reduced turbidity, inducing chlorophyll a and consequently primary productivity in the lower river basin and upper estuary. The coastal plain attenuated the increasing downstream material fluxes, but also curbed suspended sediment and TN fluxes, across the lower river/upper estuary. In contrast, TP and PO43− fluxes increased sharply across the coastal plain. This was influenced by natural and anthropogenic P inputs from soils and run‐off to the river channel. These exceeded the retention capacity of the coastal plain and result in a high export efficiency of this nutrient to the sea.