Which preposition to use with cydnus

during Occurrences 1%

Having surmounted every obstacle thrown in his way by the artifices of the Greeks and the power of the infidels, he had penetrated to the borders of Syria; when, bathing in the cold river Cydnus during the greatest heat of the summer season, he was seized with a mortal distemper, which put an end to his life and his rash enterprise [g].

Which preposition to use with  cydnus