plunge pool vs shoal

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. 

shoal

noun
  • A shallow in a body of water. 

  • A sandbank or sandbar creating a shallow. 

  • A large number of fish (or other sea creatures) of the same species swimming together. 

  • Any large number of persons or things. 

verb
  • To become shallow. 

  • To arrive at a shallow (or less deep) area. 

  • To collect in a shoal; to throng. 

  • To cause a shallowing; to come to a more shallow part of. 

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