Each stack has 2 pallets and 4 interstitial 2"x4"s (underneath and between the pallets), topped by 6 or 8 rocks ranging from fist to head sized. Based on pallet experience in pond #2, I think that will keep them down; I know I can redo them from shore if they float up after it fills.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
[Linked Image from i.pinimg.com]