River rock is an economical decorative stone that tends to be smooth and approximately 1 ½ inches in size. It is available in a mix of muted colors including tans, reds, whites, blues and grays. This stone works great in areas where mulch would wash away, for example a steep slope that needs to be covered with material or to prevent erosion in areas that have a high flow rate of water, such as under a gutter drain.