Possible Clues:

• Part of popular northern river
• European river known for disastrous floods
• It flows through 55-Across
• River flowing through Florence and Pisa
• A refusal to hold red flower in 17
• River connected by canal to the Tiber
• Rook found in an old European river
• Part of the view in 'A Room With a View'
• The New Yorker cartoonist
• River that rises in Monte Falterona