estuary vs rivermouth

estuary

noun
  • An ocean inlet also fed by fresh river water. 

  • A coastal water body where ocean tides and river water merge, resulting in a brackish water zone. 

rivermouth

noun
  • The mouth of a river (where it flows into a lake or sea) 

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