It is joined by the Ucayali, a great navigable river with a course of 600 m.
The country between the Huallaga and the Ucayali, traversed by the Eastern Cordillera, is called the Pampa del Sacramento, and is characterized by extensive grassy plains.
The forests drained by the Maranon, Huallaga and Ucayali form the northern portion of the Peruvian montana.
Surveys were completed in 1909 for an extension of the Oroya line from a point on its Cerro de Pasco branch eastward to the Ucayali, and another transandean line frequently discussed is projected from Paita across the Andes to Puerto Limon, on the Maranon-a distance of 410 m.
The rivers forming this system are the Maranon from Puerto Limon to Tabatinga on the Brazilian frontier (484 m.), the Japura, Putumayo, Javary, Napo, Tigre, Huallaga, Ucayali, Pachitea, Jurua, Purus, Acre, Curaray and Aguarico all navigable over parts of their courses for steamers of 4 to 8 f t.