O Brother, Where Art Thou? (2000)

Continuity mistake: There are seven Wharvey Gals. Three on the stage singing, three with the wife, and one that she's holding. One of the girls even says that there are seven of them. Yet in the end, when Everett and his wife are walking down the street, there are only six. One that she is carrying, and five following.

Pappy O'Daniel: I'll press your flesh, you dimwitted sumbitch! You don't tell your pappy how to court the electorate. We ain't one-at-a-timin' here. We're mass communicating.

Trivia: George Clooney (Everett) was going to sing "Man of Constant Sorrow" for the film but his singing voice was very poor so he ended up lip-synching the songs instead. He said "I'm not my aunt [referring to the late singer/actress Rosemary Clooney, best known for her role in "A White Christmas" (1954)]. I decided it would be easier to just do a passionate lip-sync." He was so nervous that the tapes of his singing would get out that he returned to the studio to ensure all the evidence had been erased. The musical director of the film confirmed this but said "George is a very good singer but that style of music is very difficult and one almost has to grow up singing it in order to sing it convincingly. George did a really good version of the tune but it wasn't as good as he wanted."

Question: The only way that the valley could be flooded by rampaging water is that a dam was broken up river. Otherwise the water rises very slowly. Does the movie explain this? If so I missed it. In which way was that river flooded?