Organic matter dynamics in alpine rivers are highly sensitive to climate change, yet long-term trends in riverine organic matter (ROM) remain poorly. Using satellite and in situ observation data and machine-learning models, we discovered that midstream regions of the Yellow River's headstream show higher and increasing ROM linked to wetter conditions, richer soils, and greater vegetation, but climate change is projected to reduce ROM in the Yellow River's headstreams by the end of the century.