The upper Maranon traverses the department from S.
At the junction of 2 D the Cauches with that river, that Ecuadorean line descends the Chinchipe to the Maranon, and the Peruvian ascends to a point where it is intersected by a line following the eastern Cordillera northward to the head-waters of the Caqueta, or Japura, which forms the northern boundary down to the Brazilian frontier.
No river, except the Maranon, breaks through it either to the east or west, while more than twenty coast streams rise on its slopes and force their way through the maritime chain.
It then continues northward, separating the basins of the Maranon and Huallaga; and at the northern frontier of Peru it is at length broken through by the Maranon flowing eastward.
The eastern range is cut through by six rivers in Peru, namely, the Maranon and Huallaga, the Perene, Mantaro, Apurimac, Vilcamayu and Paucartambo, the last five being tributaries of the Ucayali.