Possible Clues:

• River with plague at the bottom
• River disease is most serious
• River meeting city on the Danube measuring the most fathoms
• Flower disease that's most difficult to fathom
• River trial is most demanding
• Like a bass' voice
• Troublesome creature under river most hard to get at there?
• Taking longest for the plumber to get to
• Most mysterious
• Exercise in river and street is hardest to understand