yeah that was worrying me too - i had a natural pool guy pop by today to see what he thinks and he said epdm the inside of the pond. (like you) but instead of another row on top, fixing the epdm to an aliminium band that runs inside the pool, below the current wall top. (this is possible because the waterfall doesn't allow for pool to be completely full anyway). sigh - the bad news is COST! yikes. double what it cost to build the darn thing in the first place.