The source, fountain, spring, or beginning, as of a stream or
   river; as, the head of the Nile; hence, the altitude of the source, or
   the height of the surface, as of water, above a given place, as above
   an orifice at which it issues, and the pressure resulting from the
   height or from motion; sometimes also, the quantity in reserve; as, a
   mill or reservoir has a good head of water, or ten feet head; also,
   that part of a gulf or bay most remote from the outlet or the sea.