A rivulet is a small stream. A rivulet is to a river as a baby is to an adult.
There are several ways to remember this word. One is the similarity of its sound to "river." Another is the suffix -let, which indicates small things. For example, a piglet is a small pig or baby pig. A 300-pound pig probably wouldn't drown in a tiny rivulet, but a piglet should stay clear. Another word for rivulet is "streamlet."