rill vs rivulet

rill

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

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

rivulet

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

  • Perizoma affinitatum, a geometrid moth. 

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