The line follows the Javary to its junction with the Amazon, and runs thence north by east direct to the mouth of the Apaporis, a tributary of the Yapura, in about 1° 30' S.
The Colombian claim included the left bank of the Amazon eastward to the Auahy or Avahy-parana channel between the Amazon and Yapura, whence the line ran northward to the Negro near the intersection of the 66th meridian.
The principal rivers of the Amazon system are the Napo, the upper part of which forms the provisional boundary line with Ecuador, the Putumayo or Ica, and the Caqueta or Japura (Yapura), which flow from the Andes entirely across the eastern plains, and the Guainia, which rises on the northern slopes of the Serra Tunaji near the provisional Brazilian frontier, and flows with a great northward curve to the Venezuelan and Brazilian frontiers, and is thereafter known as the Rio Negro, one of the largest tributaries of the Amazon.
West of the Negro the Amazon receives three more imposing streams from the north-west - the Yapura, the Ica or Putumayo, and the Napo.
The first was formerly known as the Hyapora, but its Brazilian part is now called the Yapura, and its Colombian portion the Caqueta.
Several very easy and almost complete water-routes exist between the Yapura and Negro across the low, flat intervening country.
Barao de Marajo says there are six of them, and one which connects the upper Yapura with the Uaupes branch of the Negro; thus the Indian tribes of the respective valleys have facile contact with each other.