To add what Tadpole suggested I always add a little extra in liner to add a fold at the bottom of the waterfall to keep it from being to taught when you set your boulders. The same goes for your biofalls if you are using one and for your skimmer. I also add a big fold at the bottom of the skimmer in case you have to replace or reset a different skimmer. This way you have plenty of liner and you don't have to rebuild the whole pond with a new liner.
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