Set Large Boulders
Use rocks and boulders of different sizes, from 14 inches in diameter to as large as you want to go, keeping in mind the size of your creek bed and your yard. For a more natural look, dig out a depression for the boulder to rest in; remember to showcase the rock's best side. Then, pack more soil around the sides of the boulder to keep it from moving. Rather than lining the sides of the creek bed with rocks, create clusters of different size boulders along the river's edge for a more natural look.
If you want to facilitate the growth of moss on the rocks, spray them with buttermilk and place the rocks in a moist, shady area.