Winterkill is a pretty complex problem but can be simplified a little.
Once ice cover is on a body of water, sunlight gets really restricted in the water column. Hence, algae struggles to survive, much less thrive. So the dead algae along with any other dead organics begin to decay. Aerobic decay consumes oxygen and leads to the winterkill.
Some of the trickier parts of how they contribute to the problem are length of time the water has been iced over. Thickness of snowpack, depth of water (more water = more volume for holding dissolved O2 prior to icing over), lots of small things influence this.
There are parts of Sak where winterkill take place. I've talked with fisheries staff in the past where there have been some issues around Little Knife Bay, so I'm sure there are other places as well.
This same kind of thing also happens in the summer, but water temp is a big factor in the summer dieoffs. Warmer water holds less O2, so when the sun goes down or gets hidden behind a cloud the rate of O2 consumption by decay outstrips the production of O2 by algae/plants in the water. Granted, this is a much larger problem in places like Georgia.
