Curious - what's the legality of roping off swimming areas?

[subzero350]

I was sport fishing the Barbee Lake Chain yesterday and came upon a roped off swimming area on Kuhn Lake that extended at least 100' out into the lake, flanked on one side and the end by an "L" shaped pier bearing signs that said "no trespassing". The roped off area of water measured approx 100'x30' or so (I had no way of measuring, so these are just approximations based on my sight observations).

I was not interested in fishing within the roped off swimming area (it was unoccupied at the time) but it peaked my curiosity. If the lakes are public, how can private property owners "rope off" areas extending out into the lake from the shore line and make those areas of the lake off limits to the public? Is this legal?

[morrison]

Areas cannot be roped off without a permit. We are seeing more property owners posting their riparian waters. However, individuals can make a roped off area but are required to leave an opening large enough for boat traffic, generally speaking that is 10 feet wide.