Depth could be dependent on your location and the runoff area you are draining.

Regarding location, in an arid area you need to worry about droughts. In cold climates, you need to think about ice caps and possible aeration concerns. I'm in North Central Texas. Rainfall has been very sparce this year. Mine is about three ft. low already and by August I'll be worried a usual. Heck, sometimes they dry up around me. I wish I had a greater runoff area for the rare good rains.

On the other hand, one of my neighbors lost his dam last year in a big rain. It just couldn't handle the runoff.

Maybe others can chime in regarding ice. I rarely see it.