Roxy, Mizz Rose

Actually, if the Impala went into the river, the Winchesters would have to hang around Smallville long enough to get the car all cleaned up and running again, which would give them plenty of time to search the area for things that might need killing. Maybe Lex and Dean (and Sam?) are after a dangerous mutant (not realizing that he's a mutant, exactly, but knowing that he's something very strange and very dangerous) and run across Clark, who's also trying to deal with the guy before he kills/eats/petrifies/whatevers anyone else, and that's how the Winchesters see enough to realize -- or to confirm their previous suspicions, if they've had them -- that Clark Kent isn't your average, ordinary farmboy.