In Chapter 12, Hank personifies the sight and sound of running water:
We dreamed along through glades in a mist of green light that got its tint from the sun-drenched roof of leaves overhead, and by our feet the clearest and coldest of runlets went frisking and gossiping over its reefs and making a sort of whispering music comfortable to hear; and at times we left the world behind and entered the solemn great deeps and rich gloom of the forest.
Hank gives the runlets multiple human qualities, specifically the ability to frisk, gossip, and make music. It's as if the stream is playfully moving over its rocky bed, engaging with the obstacles in its path. Furthermore, the stream is personified as if it is producing music through its movement, creating a soothing and comforting atmosphere for those who hear it.
By personifying the runlet in this way, Twain not only adds a sense of liveliness and charm to the scene, but also invites the reader to connect with and relate to the natural world. This personification helps to create a more vivid and engaging sensory experience for the reader, as they can imagine the stream as a playful and chatty companion on the journey.