Possible Clues:

• River between Poland and Germany
• Czech/Polish river
• A German river flowing through a city called Frankfurt
• River finished with a hundred-fold increase in content
• Ruddy duck back in river
• Smelly-sounding river
• River flowing from Czechia through Poland and Germany to the Baltic Sea
• River forming part of the border between Germany and Poland
• Ordinary German word for the river
• Polish river, or German