The speaker addresses a river and treats it with mock disrespect, calling it "senile" and describing its bed as a floor of "pointless garbage": the thin skeletons of animals and granite left over from a mountain that was worn away long ago. The river seems to have swallowed whole landscapes, and the phrase "lusty banquet of your middle age" turns erosion into a greedy feast.
Then the poem turns. After all that deep time and slow destruction, the speaker says the river is "less than the least silence" of a spellbound child. That young person takes in "a nascent universe / with each unlikely breath," so a single moment of wonder outweighs millions of years of geology.
The tone is irreverent and a little teasing, but the ending is full of awe. Judging by the imagery, the speaker values fresh attention and curiosity more than sheer age or size. The mood moves from decay and heaviness to a quick, bright sense of beginning.