To assess the role that erosion processes play in governing the character of particulate organic carbon (POC) discharged from small mountainous and upland rivers, a suite of watersheds from Oregon, California, and New Zealand was investigated. The rivers share similar geology, tectonic setting, and climate, but have sediment yields that range over 3 orders of magnitude. The ¹⁴C age of the POC loads is highly correlated with sediment yield. Carbon isotope mass balances reveal that the rivers carry bimodal mixtures of modern-plant- and ancient-rock-derived OC. At lower yields, modern plant OC dominates the material delivered to the river by sheetwash and shallow landsliding. With increasing yield, a progressively larger part of the POC is contributed directly from bedrock erosion via deep gully incision. Our results support the inference that active margin watersheds are important sources of aged POC to the ocean.