Pond turnover means the water at the bottom of the pond comes to the surface and the surface water heads to the bottom. It is not unusual to get turnover after heavy rain events if your pond has developed a thermocline - portion of the water column where the temperature of the water changes significantly. The water at the bottom of the pond generally has poor dissolved oxygen - fish will tend to hang out where the DO is best. When the pond turns, the fish are hanging out in the upper part of the water column and when the low DO bottom water hits the upper water column the fish basically suffocate. If you read enough on here you will see that aeration is a popular topic. Aeration, when done properly, provides a more consistent DO level throughout the water column. I had a turn over fish kill after Harvey - wasn't bad but it was enough for me to do the research and figure out what happened.
That's the Cliff Notes on pond turnover. Having said all that - I do not know if turnover is that big a problem in shallow small ponds such as yours. Max depth of 6' is not that deep so there isn't much water column in which to develop a thermocline and get DO stratified especially if your max 6' depth is a small area. But, as noted, I don't know so maybe someone else on here can tell you.
There are trees that are actually bad for fish for various reasons - black walnut and oak for example. I just learned this myself last week on this forum so no expert but you might do some research. A small pond such as yours with small water volume may be more susceptible to any type of contamination - maybe even something the storm blew in that has nothing to do with your place.
Hope this information helps or at least gives you a place to start researching. Good Luck..................
