Your rug needs a backing with a regular series of holes in order to drain the water that collects between the pebbles. Glue together two layers of perforated drawer liners to give them extra strength and use this as a base for your pebble rug. For larger rugs, choose a stronger material, such as woven door mats.
If you're lucky enough to live next to a river or brook, you may have a ready supply of free pebbles. Check with the landowners before removing buckets of stones, though. If you don't have a ready source of free rocks, get yours at craft supply stores or landscaping supply companies. Pick through the rocks and choose those that are approximately the same size and shape. Add a few odd-shaped rocks for variation and interest but keep them all about the same height.
Your rug will need waterproof adhesive, no matter where it will reside, but the space you put the rug will determine the glue you need to use. If you're making an indoor rug for a hearth or bath mat, use silicone glue. It's easy to use and will withstand moisture from a steamy bathroom. For outdoor rugs, use something stronger, such as landscape adhesive. This special glue is designs to stick rocks to other materials, so you'll have the best chance of keeping your rug in one piece, no matter what the weather.
Place your river pebble rug in a prominent place in your home to show off its natural good looks. Use it as a doormat to greet your guests. Set the rug in front of a fireplace to accent the brick or stone. A river rock rug is a natural in front of a shower stall as the smooth rocks will welcome your feet as the water drops through the mesh below. Set the rug at the bottom of pool stairs or below the steps to your hot tub.