The Pilcomayo river begins in the Bolivian Andes and flows eastwards across the Chaco region, for 1,100 km before joining the Paraguay river. Today this river marks the border between Argentina and Paraguay. It is a typical lowlands river that overflows in the rainy season. In the dry season which is often characterized by the a complete absence of rains, the river loses its water by infiltration and evaporation. The high lateral mobility capacity of the Pilcomayo river has resulted in the generation of the largest functional alluvial fan in South America. This fan covers a surface of210.000 Km2, which is characterized by a numerous abandoned river channels. The sediments are composed of very fine quartz sand originated in high altitudinal upper basin in Bolivia. The episodic obstruction of the colector in the flat fan leads to frequent avulsions and abandonments of ancient channels and the generation of new ones.The development of the fan occurs along a sequence of climatic changes in tropical South America