Rivers and estuaries are major pathways of microplastic (MP) from terrestrial areas to marine ecosystems. Despite this, knowledge on the transport dynamics and fate of MPs in riverine and estuarine waters is limited, but crucial in order to mitigate the problem and to close the mass balance of plastic debris in the oceans. Via ex situ laboratory settling experiments emulating the Msimbazi River and estuary, Dar es-Salaam, Tanzania, we demonstrate that flocculation and subsequent settling of positively buoyant MP (HDPE, 63-125 µm) and fine-grained suspended sediment (