Thanks for all the replies.

But I'm not trying to seal liner together. I have a very large flat rock
that is mostly buried. The downslope edge hangs out a little over a 6"
drop. I want to dig back further under the rock to form a small pool under
the drop. The stream would flow over the rock, fall into the pool below and
then continue on its way. The pool would have a liner bottom.

My basic question is, "Is it possible to make a waterproof join between the
pool liner and the immovable rock, and if so, how?"