This poem is too loaded with imagery to give just one example, so I'll hand you a few. Look first at the initial stanza: "...my hair was still cut straight across my forehead" allows the reader to mentally see the author's face with its military-straight bangs.
Other images that "stick" in the reader's mind include the swirling eddies of the river, the monkeys making noise overhead, the paired butterflies yellow with August, and the overgrown mosses. Each of these elements can be easily mentally pictured by the reader, and each has its own significance.
For instance, the paired butterflies symbolize mating or coupling, a ritual that the author laments missing. The howling monkeys seem to indicate pain, sorrow, or mourning, while the overgrown mosses indicate the excessive passage of time, which the author also laments. The mental pictures provided by this poem are vivid and stark, and perhaps its imagery is what makes it so effective.