River rocks are shaped by hundreds or even thousands of years of  rushing water giving them and inherent sense of longevity and natural time.  I really enjoy using river rock and pebbles in my landscape design.  They add texture and contrast as well as serve as borders or ground cover.   At the Portland Street of Dreams this year several of the houses had incorporated river rock.  I especially like how in some of these beds the river rock ends organically by tapering off.