estuary vs river basin

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. 

river basin

noun
  • An extent of land where water from rain or snow melt drains downhill into a river or series of rivers. 

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