I keep reading that deeper ponds are less prone to fish kills.
This is not always true and as usual - it all depends. When you read something you have to consider the source, the author's knowledge, don't take things out of context and don't assume.
The statement "deeper less prone to fish kills" is much more true for ponds in ice cover conditions and winter fish kills rather than during mid-summer conditions. Numerous things affect fish kills in summer such as surface to depth ratio, wind exposure, age of the pond, productivity of the pond, water clarity, cloudy weather (lack of sunshine), cold rain, pond water retention time, amount of submerged vegetation, on and on with added factors.
Steve in his post above has his information backwards. Steve said ""I believe it has to do with the fact that warm water is more dense
and sinks to the bottom in the winter."
Warm water is less
dense NOT more dense than cold water. In reality,, Less dense "lighter" warm water "floats" on the heavier colder more dense water. As the upper warm water cools it becomes heavier, it sinks and is more easily mixed downward by wind, cooling, and convection currents. This can cause deeper less DO water often with dissolved toxic hydrogen sulfide (H2S) to circulate upward (mix) into the "good" water above. H2S is very toxic to fish. This mixing with a double threat of no DO & elevated H2S can degrade the entire water column enough to suffocate and kill vulnerable fish - invertebrates. H2S concentrations vary from 0 to high due to the duration of lack of dissolved oxygen. If DO is present - H2S will not form. H2S forms only in areas where no DO is present.
Continuing - In winter, ponds especially those with ice cover
, can have a THIN warmer layer at the bottom (see later). In winter ponds that do not develop ice cover, these ponds often will regularly
or often completely circulate and not maintain a warm bottom layer due the water column of 39F-42F, convection currents, and wind action mixing. Thus the thin warmer bottom layer can frequently get disrupted and diluted into the upper colder water. The winter thin warm layer in non-iced ponds is often very temporary or non-existent depending
on the individual pond conditions.
This warm bottom layer during ice cover is not very thick as in a maximum of 2" to maybe 5" thick above the bottom & mostly 2"-3" thick as by my measurements. This thin warmer layer is due to a mild geothermal affect of the bottom mud "giving back" some of the bottom layer heat that was absorbed during summer. As the winter progresses the geothermal affect is lessened. This warmer layer by my measurements is usually only 1F to 2F warmer than the 39F water above it. Keep in mind that this warmer water is lighter less dense so it "wants" to rise into the 39F water, and it quickly gets diluted and cooled by the massive amount of 39F water above plus the geothermal affect is not intense, but mild.
The warm thin layer can get easily mixed, diluted, and somewhat blended into the upper 39F water by bottom oriented fish activity. In some ponds with sizable underground spring fed water flow the warmer layer can be thicker and larger due to the constant flow or influence of spring ground water often being near 50F.
Wouldn't a deeper pond have a greater difference in temperature between surface and bottom that might make it more prone to turning over?
No, No to more prone to turning over, And Yes possibly more prone to a fish kill due to a larger volume of bad deep water compared to upper warm layer volume. Here is where surface to volume ratio comes into play.
the difference in temperature between surface and bottom the greater
the resistance is to mixing between the two layers. In other words the greater the difference in temperature the less the two layers will "want" to mix together. As the temperature difference increases the more energy it takes to combine or mix the two temperature layers together. Energy as in sun (heat, thermal), wind, mechanical.