Effective management of sediment fluxes in aquatic systems involves, in part, the identification of catchment sediment sources. Lake Wivenhoe (LW), the largest water storage in south-east Queensland, serves two important roles: it supplies 80% of the drinking water to the region and acts as a major flood mitigation feature for the city of Brisbane. Highly developed subcatchments in LW have resulted in declining waterway health, with sediment movement from the catchment to LW of major concern. Although there is considerable hydrological information, only limited data exist on sediment and nutrient fluxes. A detailed lake sediment (128 samples) and reconnaissance catchment soil sampling program (89 samples) was undertaken. Geochemical, Nd–Sr isotopic and statistical analyses were used to identify major sources of sediment to LW. A purpose-built Bayesian mixing model was then used to quantitatively estimate the proportion of sediment from major catchment sources. Approximately 36% of the LW catchment delivers the majority of sediment; enrichment factors for the three major sediment sources (dam to catchment ratio) range from ~2 to 5. The Esk Formation is the major sediment source comprising ~10% of catchment area but contributing 50% of the sediment and 33% of the total phosphorus delivered to LW.