Oh I see. I am familiar with that phenomenon but didn't think about cold rain in the summer. (We see this here in early fall.) However if we are talking the south and temps we have had I suppose any rain would be cold! [/quote]

Most thunderstorms that produce hail carry with it some very cold rain in the summer. The temperature of that water immediately cools the top layer and allows the bottom low DO layer to mix. I'm talking about a "big" rain +2 inches or more normally. In may case it was also accompanied by an algae bloom kill. The rain is the "final nail" that did them in.