[steveegg]

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.

[geopyg]

"It's water temps that do it...."

The life cycle of a lake. I've joked that God made up the rules for physics like cold fluids (air/water/etc) are heavier than warm fluids, solids are denser than liquids, etc.

UNTIL the first spring - when he realized that the solid water (ice) sank and killed all the vegetation and critters in all the lakes. Oops!

"Okay - all solids, EXCEPT water, will be denser than their liquids."