Hi Fuzzy.
The cellophane is glued down with PVA - just along the edges. The piece I glued down is larger than the visible river area, so the glued edge becomes invisible with the land pieces glued on top. I then 'blend' the join with gap filler, sand, paint and static grass.
I'd love to play a game or two if I'm ever in Melbourne. Likewise, let me know if you're ever in the Wellington / Kapiti region.