estuary vs rivulet

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. 

rivulet

noun
  • A small brook or stream; a streamlet; a gill. 

  • Perizoma affinitatum, a geometrid moth. 

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