What category does the following fit in? "tolstoy writes well of despair; he had plenty of it during his lifetime." authenticity universality subject matter aesthetics

What sound device is used in the following excerpt from how the animals lost their tails and got them back traveling from philadelphia to medicine hat by carl sandburg

Danny went for a long bike ride he covered at least forty miles

[he] let his gaze wander to the swirling water of the stream racing madly beneath his feet. a piece of dancing driftwood caught his attention and his eyes followed it down the current. how slowly it appeared to move! what a sluggish stream! what contrasting words are used to describe the stream in this passage?