The Collaborative International Dictionary of English v.0.48:

Down \Down\, prep. [From Down, adv.]
   1. In a descending direction along; from a higher to a lower
      place upon or within; at a lower place in or on; as, down
      a hill; down a well.
      [1913 Webster]

   2. Hence: Towards the mouth of a river; towards the sea; as,
      to sail or swim down a stream; to sail down the sound.
      [1913 Webster]

   Down the country, toward the sea, or toward the part where
      rivers discharge their waters into the ocean.

   Down the sound, in the direction of the ebbing tide; toward
      the sea.
      [1913 Webster]