The Guaraní Under Spanish Rule in the Río de la Plata, Barbara Anne Ganson, pp. 23, Stanford University Press, 2006, ISBN 9780804754958, ... Juan Diaz de Solis entered the area in 1516 and discovered the river the Guarani called Parana-guazii, the yet-to-be-named Rio de la Plata (River of Silver) ...