The Bayou is a vast stretch of jungle like forest and swamps covering the entire southeastern United States. Following World War 3, the radiation and fallout from the detonation of nuclear weapons rained down onto the southern states. This in turn amplified the growth rate of plants in the region and soon enough the South would morph into a pseudo rainforest as a result. The people that still lived in the area were also mutated by the radiation, becoming grotesque and simple minded (though some did manage to remain sentient and intelligent and often venture out of the Bayou into the American Wastes.)
The Republic of Texas often sends groups of people on expeditions into the Bayou to collect items from before the war. Guns, vehicles, various items from a time long gone.