A river cuts through stone, and the poem watches it happen with quiet awe. The water is described as having "steel flow," which makes something soft and moving feel hard and unstoppable. It pierces rocks, but it doesn't simply destroy them. It shapes them into stars, so the damage becomes a kind of beauty.
The sun and the moon, the most powerful things in the sky, can't match the river's sharpness. The speaker seems to admire the river's patient force more than any grand, distant light.
The last lines are the most striking. The rocks wail as they are worn down, and that sound becomes the river's song. Judging by this image, the poem suggests that suffering and change can be turned into something that sounds like music. The mood is calm but powerful, mixing admiration, a hint of sorrow for the rocks, and a sense that endurance can create something lasting.