The Radiocarbon Reservoir Effect (RRE) involves the uptake of older carbon into aquatic and terrestrial food webs. It can increase radiocarbon ages on consumers of aquatic foods by decades to centuries. While RRE's have been thoroughly investigated in many parts of the world, the RRE of North America's largest river is unknown. This lecture will discuss the RRE in the Mississippi River floodplain and examine the possible causes and implications of differences between terrestrial herbivore and aquatic fish bone samples from two archaeological sites.