Famous river (6)
(Other definitions for amazon that I've seen before include "Female warrior of Greek myth" , "Mythical woman warrior and river" , "S. America's longest river" , "Great river of S America, or a tough woman" , "Encourage, ask" .)
(Other definitions for tigris that I've seen before include "flower of Iraq" , "Great river flowing from Turkey through Iraq" , "Euphrates companion river" , "Major tributary of the Euphrates" , "River of Turkey/Iraq" .)