This is another sad/happy memory symbolism.  Dream with me a moment. Life's water slowly passes by while it deposits a little dirt on the bottom and wears off the sharp edges of every stone it passes.  Nothing seems to be living here, but if you flip over any flat rock you will find life under it, afraid of the light.  If you move your focus back you will see reflections in the water.  Reflections of the sun passing thru casting shadows on your memories.