Maybe Madeira River is indeed not so famous, but it is a verry important river, the first tributary of the Amazon in terms of length and discharge. (Twice the discharge of Mississippi!) It's why I wanted to put it here.
For the Sao Fransisco, I hesitate with 3 or 4 other rivers, but finally decided not to keep it, even if I know it's necessarily an arbitrary choice. I hope you still enjoyed it!