If you put soil anywhere near where you are channeling the run off from the waterfall, the soil is going to get washed out into the "pond area" - Have soil surrounding a gravel/rock bed and have the water channeled to flow over the rocks. Otherwise you are going to go crazy netting out the soil from the water and fixing bald patches. This is why I made the "stream" out of the plastic soda bottles so it would curve down for depth and used the liner over it with rocks to hold everything in place and disguise what I did underneath. I used sand on each side of my stream rather than soil but if I had just sand in the stream itself it would have all been washed out into the pond area with in moments of turning on the water.