You could also stack thin "slate" type rock from large on the bottom to small. If you use wood, be sure to use wood that isn't going to rot easily or you'll just have mush in no time. I use a lot of ropy lava rock, which makes great falls, but isn't usually available unless you can collect it nearby. It's also extremely heavy. I have one falls of petrified wood. Take your time and have fun with it. I don't know where you live, but if you can get out in the mountains or hills, look for neat stuff to use.
Last fall when I was in the Nevada desert with my son, who is a geologist, I found some great stuff I was going to use as "paving stones" around a pool. He said it would be very nice, but many of the wonderful flat stones I'd collected were full of aresenic, and I ought to be careful about using them. There's nothing like having an expert in the family to spoil your fun.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus