There are two Frankfurts in Germany. The one lies on the Main, the other on the Oder. The reason why Frankfurt am Main is referred to as Frankfurt am Main and Frankfurt (Oder) as Frankfurt (Oder) stems from the fact that there are two. Assuming that people will always think of FaM when Frankfurt is mentioned and not of F(O) seems pretty unfair towards the Oder-Frankfurters. I would rephrase the question to "Which river flows through Hessian Frankfurt?"
Fun quiz! Regarding the question about the deepest lake in North America: I typed "Great Slave" and it wasn't accepted, so I gave up on it. Since "lake" is in the clue, would you consider accepting just the name, without requiring that "lake" be typed as well?