Pondlady sez:
You will have disastrous results if you try to make a pond bigger by seaming liners. You simply cannot seam liner in the field. Try building a second pond right next to the first with a soil wall in between. Use that space for a walkway or a waterfall, maybe two. You will need a second pump, of course. But the two ponds will look like one. This is a much cheaper and easier way to get a bigger pond.
Here's one that I did.
