The manufactured beaches that seem to have best success in the ADK park are those that are made on extremely gentle inclines. In fact the deeper you go, they get even less steep to almost level. Then at that deep edge of the swimming area are larger stones and gravel that dams the sand flow.

If I were to have the resources of such a large lake, I would have a beach area excavated with this in mind, then back fill with first landscape fabric, then several inches #2 crusher run, and on top at least a foot of good sand. I would avoid landscape fabric above the waterline where kids are likely to dig.

Even so, it seems sand needs to be replenished on the shore every so often. But in the ADK good washed coarse sand is plentiful.