West of the Rio Negro, the Solimões River (as the Amazon's upper Brazilian course is called) receives three more imposing streams from the northwest—the Japurá, the Içá (referred to as the Putumayo before it crosses over into Brazil), and the Napo.

This multiple confluence (resembling a very elongated delta) complicates the measurement of the length of the Caquetá-Japurá, which varies, depending on the method used, from 2200 to 2800 km, especially because the boundaries between the basins of other tributaries and sub-tributaries of the Amazon system are unclear in this flat, flooded, and swampy area.

[citation needed] Slave raids against the indigenous people of the Caqueta/Japurá River valley had persisted for at least 100 years prior to Roger Casement's investigation of the Putumayo genocide in 1910.

While citing a book published by English lieutenant Henry Lister Maw, Casement noted that these slave raids had been continued by Brazilian and Portuguese men.

The Boras people were primarily dedicated to rubber extraction around the stations of Abisinia, Santa Catalina and La Sabana[9] correspondingly managed by Abelardo Agüero, Arístides Rodríguez and his brother Aurelio.

[15] The river is home to a wide variety of fish and reptiles, including enormous catfish weighing up to 91 kg (201 lb) and measuring up to 1.8 metres (5.9 ft) in length[citation needed], electric eels, piranhas, turtles, and caimans.

[citation needed] The 19th-century Brazilian historian and geographer José Coelho da Gama e Abreu, the Baron of Marajó, attributed 970 kilometres (600 mi) of navigable stretches to it.

[6] The Baron of Marajó wrote that there were six of them, and one which connects the upper Japurá with the Vaupés branch of the Negro; thus the indigenous tribes of the respective valleys have easy contact with each other.