What is the central idea of the poem "The River" from The Water-Babies by Charles Kingsley?

Charles Kingsley's novel The Water-Babies is peppered with songs and poems, some of which are original to the novel. The poem about the river is original (written by Kingsley himself) and actually appears untitled within the story. You can find it near the beginning, as Tom is starting his journey. Outside of the novel, the poem appears under the title "Inland Waters: Highlands Song of the River."

It's a short poem, just three stanzas long. Its central idea is that a river can be symbolic of both cleanliness and filth, and both sin and purity of the soul. 

More specifically, a river can be a place for playing, laughing, dreaming, and bathing: a pure place where a mother and child can belong. It can reflect the cleanest, most admirable aspects of the human spirit.

At the same time, a river can be dark, dank, foul, and slimy: a place where a mother and child should naturally feel repulsed. These portions of the river reflect the sinful, unclean aspects of humanity.

The point is that the river is defiled in some places and undefiled in others. It's complex and changing. The same is true of humanity. We're debauched at times, pure at other times.

The author seems to hint that sin and filth are related to greed and the accumulation of money. "Baser and baser the richer I grow," the river says, meaning that it gets more disgusting and unclean as it gets "richer." However, the poem seems to have a hopeful message overall: the first and last stanza are about the purity of the river, while the second stanza is about its filth. This structure allows the poem to end on a pure note. Toward the end of the final stanza, the river cries out this description of itself: "Like a soul that has sinned and is pardoned again."