The river curling softly by whispers and dimples round it's quiet gray stones what does it mean

It means that the River Flows in a curving direction making soft sounds like whispers and small undulations or irregularities of its surface create by the collision against its immobile gray stones at its bottom.

Curling = Flows in a curving direction.

By Whispers = making soft sounds like whispers

Dimples = small undulations or irregularities of its surface

Quiet Stones = immobile gray stones at its bottom.