How does Richard Hugo use imagery in For Jennifer, 6, on The Teton?

In his second stanza, Hugo focuses on describing the behavior of the river and the creatures who make their homes there. He says that "The thought of water locked tight in a sieve / brings out the beaver's greed." Although it is not clear at first that this metaphor is meant to suggest that childhood is also like "water locked tight in a sieve," the speaker's image of the river water being "violent opaque runoff" and moving, coupled with the command, "Jennifer, believe / by summer streams come clean for good," implies that the beaver's dam will unravel or come done. In this stanza, Hugo describes the temporary nature of childhood by marrying it to the temporary nature of the beaver's dam. The lines in this stanza about water moving also anticipate the poem's final last line in which "tributaries" (or small streams feeding in larger streams or lakes) "burn," and so reinforce the speaker's authority or supreme control over his subject matter.