We determined the concentrations and compositions of coarse particulate (>63 µm), fine particulate (0.1–63 µm), and dissolved (0.001–0.1 µm) organic matter collected along a river reach extending from a first‐order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from ~80 to 350 µM. The percentage of...