Possible Clues:

• Trails bordering bend in river abroad
• Name American river flowing to Atlantic
• That shows how grateful to Augustus the river is
• River of Spain/Portugal, its mouth at Lisbon
• How one may thank Augustus for the water from Portugal
• Longest river of the Iberian Peninsula
• River in Portugal
• That flows, with thanks to the boy
• A Spanish/Portuguese flower, thanks to Augustus
• Nickname useless river