I lined it with old carpet off of Craigslist, and a billboard tarp. The Structure in the back will be a cascade on one side and a river on the other. I will fill with pea gravel and plants to soften the edges as well as work as the filter.
The tarp was huge, I ended up with a 20x60 the smaller tarp 14 x 48 would have barely fit so I figured the extra would be a good call.
It is 17 mil thick and very strong, it was easy to cut. It does not strech at all.