Many American rivers puzzled navigators and cosmographers. This reaction throws light on a deep cultural layer linked to the symbology of great springs. Since Classical Antiquity and the Early Christianity, rivers have been associated with a double meaning: as obstacle (natural borderline of a collective identity) and as shortcut (a sign of the possibility of breaking through frontiers and exchanging spaces). Major concern in the European strategies regarding the penetration and controlling of the territory, the Amazon and the river Plate gave birth to a remarkable mythological complex. Their margins were to organise a number of fabulous references, such as tribes of giants and pigmies, the reign of the amazons, the resplendent mountains of Parima, the province of Omagua, the El Dorado. The role of such a fluvial imaginary was giving consistence to the unknown, helping to make the New World a more coherent and comprehensible reality.