water parting vs watershed

water parting

noun
  • A summit from whose opposite sides rainwater flows into different streams; a line separating the drainage districts of two streams or coasts; a divide; a watershed. 

watershed

noun
  • The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest. 

  • A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin. 

  • The time after which material of more adult nature (violence, swear words, sex) may be broadcast on television or radio, either one laid down by law or one contrived from convention (e.g. when children are not watching) 

  • A critical point marking a change in course or development. 

adj
  • Serving to mark a significant development, change in direction, etc. 

How often have the words water parting and watershed occurred in a corpus of books? (source: Google Ngram Viewer )