Blackwater streams are found throughout the Coastal Plain of the southeastern United States. These streams are characterized by low slopes, high summertime temperatures, and extensive inundation of surrounding floodplains. Typically lasting from winter to early spring, the long inundation period creates a multitude of instream floodplain swamps 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 for violation of the state’s dissolved oxygen (DO) standard. Generally assumed to be a consequence of increased biological activity from nitrogen and phosphorus enrichment, lowered DO may be a natural phenomenon within this system. To measure the 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 representative floodplain swamp. Water within this swamp had long travel times, with the degree of dispersion highly dependent on flow. Results show SOD rates which are generally higher than values reported for southeastern sandy-bottomed streams. Coupled with the long residence times, SOD may play a central role in determining DO levels within these instream swamps and the river system as a whole.