Water exerts about 1/2 pound of pressure for every foot of water depth on the pond bottom. So, a pond bottom has to be sealed better than a marsh bottom.
Typically it's recommended that the whole pond basin be compacted with a sheepsfoot roller in 6" lifts, for a total of 4 lifts.
Just like a bathtub, all you need is one drain hole not plugged and the whole pond will drain out.
