Erosion and riverine transport of organic carbon is an important component of the global carbon cycle, but the significance of this process for Earth’s surface carbon budgets depends on the sources of carbon being mobilised. In this study, we aim to constrain how runoff-driven erosion modulates the contribution of different carbon source endmembers, i.e., bedrock, soil and vegetation, in three forested headwater catchments in the Swiss Prealps. The sources of organic carbon are determined using an inverse model based on bulk carbon isotope signatures and the abundances and distributions of long-chain n-alkane plant wax biomarkers in suspended sediments collected over a range of discharges. Despite landcover differences and contrasting bulk particulate organic carbon (POC) signatures, the increase of soil-sourced organic carbon with discharge is similar in all three studied catchments. This apparent existence of common processes implies that export fluxes of soil organic carbon may be extrapolated to similar catchments. Overall, our analysis shows that runoff-driven soil erosion in these alpine headwater streams is responsible for the export of ca. 0.3 to 0.8 gC m-2 a-1 as POC, which represents ca. 0.1 - 0.3 % of carbon fixed by NPP. Most of this soil OC export occurs during high-discharge events. Our study also shows that despite a significant variability in isotopic and molecular POC signatures at low discharge, all three catchments show a convergence of these signatures at higher discharges. Suspended sediment samples collected at above-average discharges are hence most representative of overall endmember contributions.