The longest and most abundant rivers in South America originate in the Andes: the Amazon, the Magdalena, the Cauca and all the great western tributaries of the Orinoco and the Paraná. Many of the Andean rivers originate in the divides of the mountain ranges, in glaciers, in lagoons of glacial origin or in terrains modeled by glaciers that have already disappeared. A large part of the hydrology of the Andes is made up of lakes and marshes of diverse origin and characteristics.