There are over 1,100 tributaries that flow into the Amazon River. The three longest are the Madeira River(1), Purus River(2) and the Japur River(3).Madeira River
As the question is not precise -
[1] There is Saltwater River on the Tasman Peninsula in Tasmania.
[2] All rivers contain salts of some kind.
[3] All tidal rivers will contain saltwater -...