The speaker talks to a river that is shrinking, and the tone is part tenderness, part resignation. At first the river is told to gather silt and sand on its own breast and to "confine yourself in your own confine," as if it were being advised to accept its limits and settle into smallness. The word "quiet" suggests the river is already losing its energy.
The rest of the poem shows what that loss looks like. Water-grasses creep in and "stealthily suck your life," a quiet, almost sinister image of something being drained. A rotting boat lies on its side, and no boatman or traveller has come for a long time. The river is no longer a route for people. Only the wind's "children" remain, playing on the golden sand and singing softly, clapping with joy.
The mood is lonely and still, but not purely sad. The speaker seems to mourn the river while noticing that a new, gentler kind of life has arrived in its place.