plunge pool vs waterfall

plunge pool

noun
  • A pool that is small in diameter, but deep, usually resulting from erosion by a natural force such as a waterfall or rapids. 

waterfall

noun
  • Waterfall model 

  • A waterfall-like outpouring of liquid, smoke, etc. 

  • A flow of water over the edge of a cliff. 

  • The action of drinking from a vessel without touching it with the lips, considered more sanitary for a shared vessel. 

verb
  • To drink (something) from a container by pouring it from a height so as not to touch one's lips to the rim. 

  • To fall like a waterfall. 

  • Synonym of recycle 

How often have the words plunge pool and waterfall occurred in a corpus of books? (source: Google Ngram Viewer )