Blackwater streams are found throughout the Coastal Plain of the southeastern United States and 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.   A key influence on the DO levels within these floodplain swamps is sediment oxygen demand (SOD), a critical and dominant sink of oxygen in many river systems that is often poorly investigated or estimated in oxygen budgets.  Results show SOD rates up to 43 times higher than values reported for southeastern sandy-bottomed streams.  A key cause of these elevated SOD rates may be the distribution of highly organic soils across the river continuum.  When developing water quality models, managers should pay closer attention to the influence of SOD because when coupled with long residence times, it likely plays a central role in determining DO levels within these instream swamps and the river system as a whole.