Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OC_petro) oxidation. However, the sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar [Re]/[Na⁺] and [Re]/[SO₄²⁻] ratios, which indicate a dominance of Re from OC_petro. Despite differences in rock type and hydro-climatic setting, the three catchments have a positive correlation between river water [Re]/[Na⁺] and [Re]/[SO₄²⁻] and water discharge. We propose that this reflects preferential routing of Re from a near-surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock-organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO₂ release by OC_petro oxidation of 5.7 ^+6.6/_−2.0 tC km⁻² yr⁻¹ for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OC_petro in river sediments is large, and the findings call for more measurements of OC_petro oxidation in mountains and rivers as they cross floodplains.