This article attempted to trace the waters and animals that once existed but disappeared into the history of the Red River Delta, and how human and climate factors combined to make this happen between the sixth and fifteenth centuries. The human factor is crucial in understanding the water history of the Red River Delta. It contributed significantly to the disappearance of swamps, lakes, the building of the first state sponsored dike, the possible changes of the course of the Red River, and the plausible origin of the Thien Duc (Song Duong) river. It appears that human reclamation of the coast mainly happened after the independence of Dai Viet in the tenth century, which was marked by increasingly intensive human activities on the coast and this increased activity is testified to by the gradual retreat of the elephants from the delta. All these changes occurred in the eastern Red River Delta few centuries ahead of the western delta. The degradation of natural environment in the eastern delta seems to have caused an out-migration to the western delta between the fifteenth and eighteenth centuries, making it one of the most densely populated areas in the world.