I don't really understand it, but suddenly, once a year or so, all the water that was on the bottom of a lake will come to the top and the water on top go to the bottom. The water coming to the top drags up lots of half rotted nasties that don't smell so good. It's called turning over. I don't think all lakes do it, I've only seen it once at lake Marble Falls, where I think it happens every year, from what the locals told me.
It's water temps that do it. As the surface cools, it eventually becomes colder than the bottom of the lake, and because colder water is more dense than warm water, it sinks.
Water has different densities at different temperatures and stratifies as a result. When the water temperature is uniform the whole way down to the bottom the water is no longer stratified. When the wind blows across the top of the lake it causes it to circulate.
The following site shows lake turnover...
/www.conservation.state.mo.us/fish/ponds/laketurn/