Seasonal influences and the role of floodplain forest as source or sink of organic matter is relatively unknown for 3arge, temperate rivers. Discharge and fine-particulate (FPOC), dissolved (DOC), and total organic carbon concentrations (TOC) were measured during five sampling periods from November, 1984, to August, 1985, above and below the floodplain-forested area (1054 ha) of Burlington Island in navigation Pool 19, upper Mississippi River. Sampling coincided with autumnal leaf fall of the floodplain forest, peak flood and falling spring flood, and low-flow conditions prior to and during phytoplankton bloom. Greatest TOC transport occurred during peak flood (8.84 × 106 Kg/day) and leaf fall (7.79 × 106 Kg/day). Peak flood transport was dominated by FPOC associated with flushing of material from upland areas. Transport during autumnal leaf fall was predominantly DOC attributed to litter leaching. Seasonal DOC loads generally increased downstream except during the phytoplankton bloom when a decrease was associated with increased microbial metabolic activity. Downstream decline in FPOC and increasing DOC loads during peak flood characterized the mechanism of deposition and processing of FPOC on the floodplain. FPOC concentration was significantly correlated to discharge and DOC concentrations were higher than FPOC except for peak flood. Significant downstream changes in TOC load suggests the importance of riparian vegetation as an influence on organic matter transport in large rivers.