High amounts of macro and microplastic have been reported in rivers, lakes and seas. However, links between the observed pollution and their sources remain unclear. This study aims to clarify these links in the Lake Geneva basin by analysing each step of the local plastic life cycle. Two distinct approaches have been compared: (i) a top-down approach based on modelling socio-economic activities, plastic losses and releases into the lake, and, (ii) a bottom-up approach based on extrapolating plastic flows into the lake based on field measurements from 6 different pathways. The two approaches yield results with similar orders of magnitude and provided a first estimation of the plastic flow from land to Lake Geneva in the order of magnitude of 55 tons year−1. Preliminary mass balance of plastic in Lake Geneva indicates that the vast majority of plastic may be deposited into the sediments.