Good points Pottsy. I doubt the old banks are still tight or compacted enough throughout to hold the new water level clear around the lake. Got to be some highly potential leakage areas around the perimeter due to drying, cracking, new tree roots, poor first time compaction, lose soils, etc. I think if you fill it up 2 ft above the existing high water line the water will drop pretty fast once the inflow stops or slows.
The lake was probably built or designed to have 2 feet of free board. Thus the upper soil banks were not intended to hold water for very long; just a guess.
