There are rocks behind the liner next to the deck, they hold the liner pretty well.....the low part is actually a test I ran on total volume for the stream (lots of rain) It passed. I am actually waiting for some of the rock on a roll to come to line along the deck and will fill in where needed behind the liner. Keeps raining here, so once it stops I can finish the cosmetic stuff (on phase 1). Thanks for the comments, keep 'em comin.