billabong vs rill

billabong

noun
  • An anabranch, backwater or oxbow lake that is temporarily cut off from the main river, especially one that is only filled with water during the rainy season and can sometimes dry up completely. 

rill

noun
  • A very small brook; a streamlet; a creek, rivulet. 

verb
  • To trickle, pour, or run like a small stream. 

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