low water vs slackening

low water

noun
  • The temporary situation of having little money. 

  • The lowest stage of a river. 

  • The location of low tide in a coastal area. 

slackening

noun
  • The act by which something slackens; loss of speed, tautness, etc. 

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