Janet Galpin wrote:
I was wondering whether to plant a small willow fairly close to a
pond. It's Salix petiolaris which is a small shrubby tree which
apparently only grows to 12' or so.

I wonder though whether there's a danger that the roots could find
their way through a a butyl liner. I know they can do that very
easily to drainage pipes.

Drains are rigid material but hollow inside, the roots merely find a gap and
grow, with a rubber liner they will simply grow beneath it and maybe cause a
few extra bumps...authenticity at no extra cost!