North of Buenos Aires , the Río Paraná is one of the world’s great rivers. In South America, only the Amazon is longer; from its source in tropical Brazil to its mouth at the temperate Atlantic, its 3,998 kilometers make it the world’s 13th-longest river and longer than the Mississippi.
Draining more than 1 million square kilometers, it flows south from Brazil, forms much of the border between Argentina and Paraguay, and finally meets the Río Uruguay north of Buenos Aires to form the Río de la Plata.
For the northeastern Argentine provinces, the Paraná and the Uruguay are a great part of their visitor appeal. In the area between the rivers, known collectively as Mesopotamia, the provinces of Entre Ríos , Corrientes , and Misiones  boast natural attractions like the rolling palm savannas of Parque Nacional El Palmar ; the wildlife-rich wetlands of Esteros del Iberá , and the thunderous Iguazú Falls .
They also have cultural allure: the Entre Ríos city of Gualeguaychú  hosts Argentina’s liveliest Carnaval, though the provincial capital of Corrientes  comes a close second. Misiones takes its name from the colonial Jesuit outposts, now in ruins, that inspired the film The Mission.
On the Paraná’s western banks, the port of Rosario  and its agricultural hinterland make Santa Fe Province  an economic powerhouse. Rosario, for its part, is a city of monuments and a vigorous cultural life that challenges Córdoba ’s claim as the republic’s second city. North of Santa Fe, Chaco and Formosa Provinces  are hot, subtropical lowlands whose national parks reward dedicated, environmentally sensitive travelers; and the trans-Chaco route from Salta to Resistencia , Corrientes (including Iberá), and Misiones (Iguazú) is earning a spot on the itineraries of overland travelers.
A word on climate: Northeastern Argentina can be stiflingly hot and humid in summer, and heavy rains can occur in any season. Locals say there are only two seasons—the mild sunny winter (the best time to explore the region) and the hot muggy summer. The transition from one to the other can be sudden.