Corrected entry: That river of souls thing was made up entirely for this movie. There were five rivers in Hades but the dead were either in Tartarus or the Elysian Fields or the Asphodel Fields depending on if they were good or bad or if they had been judged yet. (There is one more area, the Isles of the Blessed, where the really good guys go who have been reincarnated three times...and of course the Plain of Judgement where they were judged).
Corrected entry: When Hephaestus is shown making lightning bolts for Zeus, he is standing straight and tall. According to Greek mythology, though, Hephaestus was a cripple.