we are going through some dry times here. my pond is down 6 or 7 inches. if we happened to get a 2 inch fast rain it would probably fill back up. i doubt this would be a problem, but what if the level goes down a few feet and then we get a heavy rain. can that cause a dangerous turnover.

iirc turnovers usually happen during a lengthy cloudy spell, right?

scott69, a turnover and a DO crash are different things, and have different causes.

A cold rain could cause a "turnover" at any pool level in a non-aerated pond. A fast, sizeable amount of cold inflow into warm water (or warm into cold) causes the water to mix or "roll over" quickly and poisons the water and fish. In the summer, the colder bottom water becomes devoid of oxygen, anoxic, and poisonous from hydrogen sulfide and other toxins released from anaerobic bacteria after all oxygen is consumed around late Spring. The thermocline virtually stops the colder water from mixing in the upper warmer water column so the toxins can be released, mixed and/or diluted gradually or more oxygen can be dissolved.

In a DO crash from several cloudy days, the pond needs a fairly heavy planktonic algae bloom that is thriving (or heavy vegetation that needs LOTS of sunlight to live). When you get a few heavily cloudy days, the algae dies, then begins to decay, consuming large amounts of dissolved oxygen and possibly suffocating the fish.

Quality, well designed bottom diffused aeration will eliminate a turnover kill, and can help minimize a DO crash potential. For a DO crash due to a heavy bloom die off, or with a high load of biomass, high horsepower surface agitation is needed to really reduce a DO crash potential.

One other thing...Warmer waters in the summer can't hold nearly as much dissolved oxygen as cooler water can.

thanks rainman. im glad you cleared that up. i am just concerned with all the dry weather and pond levels dropping in this area, once the rain starts some may have problems.

I am afraid of a sudden rainstorm causing a problem also. My pond is now at only 2/3 capacity in volume. What effect could a large rainstorm have on a pond that has only been filled about seven months? I don't have aeration yet, but probably don't have much bottom sludge.

John, your bio load is probably still pretty minimal and I doubt you'd have a problem at all.