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Blackwater streams and rivers are found throughout the Coastal Plain of the southeastern United States. These streams are characterized by low slopes, high summertime temperatures, large inputs of dissolved organic material, and extensive inundation of surrounding floodplains. Typically lasting from winter to early spring, the long inundation period creates a multitude of instream swamps and floodplain wetlands that play a vital role in overall water quality. Over 90% of the blackwater streams listed as impaired on the Coastal Plain of Georgia are listed because of violation of the states dissolved oxygen (DO) standard. Streams are listed as impaired if DO falls below a 4 mg L-1 minimum or 5 mg L-1 24-hour average. Generally assumed to be a consequence of increased biological activity from nitrogen and phosphorus enrichment, lowered DO may instead be a natural phenomenon within this system. In an effort to measure the magnitude and influence of floodplain swamps on levels of DO in the river channel, we investigated the residence time, flowpath and sediment oxygen demand (SOD) within a large instream swamp in a representative blackwater river system. Water within one swamp (1550 m in length) had a travel time between 15.5 and 27 hours, with the degree of dispersion highly dependent on flow. SOD is often a critical and dominant sink of oxygen in river systems and despite its importance, is often poorly investigated or estimated in oxygen budgets. Results show SOD rates between 0.87 - 15.84 g O2 m-2 day-1, which is generally higher than values reported in the literature for southeastern sandy-bottomed streams. Coupled with the long residence time, SOD may play a central role in determining DO levels within these instream swamps and for the river system as a whole.