The "Rio da Dúvida" was a tributary which wasn't really mapped at the time. The Brazilian government was laying out Telegraph lines at the time to map the area.
There were tribals in the area but it wasn't mapped.
Covers Teddy Roosevelt's Amazon expedition. To the comment about "weren't people living in the Amazon" - read the book. The Brazilian government was scouting and mapping the terrain for the project of connecting the coasts with telegraph lines. This was uncharted territory and the chance of not returning was high.
There were tribals in the area but it wasn't mapped.