"George" wrote in message
news:z4T4e.10070$yg7.7550@attbi_s51...
I guess you didn't read my response to Elaine:

## Sometimes my newsreader is behind.

I was waiting for someone to bring this issue up. It was discussed at
length
last fall. In an effort to see how this affects shallow garden ponds, I
conducted a little experiment, measuring the water temperature at the
surface,
and comparing it with water at the bottom (45").

Starting with autumn, let's see how the pond changed into the winter. The
water
at the surface started out in October at 42°F. The first frost and cold
wind
lowered the surface layer and it sank, while the warm water rose and
replaced it
and was chilled in turn. This circulation process (which is typical of
most
temperate bodies of water), continued until all the water reached 39.2°F,
at
this temperature water is at its heaviest. Normally, any cooling beyond
39.2°F
does not sink, instead forms a thin layer at the surface because of the
local
geothermal gradient, which will warm any water that is
below the frost line. However, with shallow ponds, the cold water can
continue
to sink (especially if the pond is not dug below the frost line), and
cause the
pond to continue to cool, and if this process continues (if the air
temperature
remains below freezing for a significant period of time), the pond can
freeze
completely to the bottom.

## Yes. I agree. That's why we try and dig them so at least one part of
the bottom is below the frost line, even here in TN.

When the weather is cold enough, especially with low wind chill, the
surface
freezes at (32°F). Freezing at the surface helps to stop the circulation,
and
the warmer
water will stay at and below the frost line. Interestingly, when my pond
started to freeze (it only froze slightly on the very ends because I use a
de-icer), the fish didn't stay on the bottom, but hovered just below the
ground
surface (my pond is 18" above ground, and 27" below ground, so they
hovered at
about 18-20" below the water surface). In other words, they stayed about
a
third of the way down in the pond. If it was significantly warmer at the
bottom, and warmth was of great concern to them, one would think that they
would
have stayed at the bottom. This was not the case.

## I was told that aside from warmth the supply of oxygen is also important
to them. But all things being equal they will rest on the bottom and expend
less energy than hovering or fighting a current, any current, all winter.
Mine stay on the bottom barely moving through the winter. In the shallow
pools they stay under the heaters.

Before the ice began to form, but while the air temperature was still
dropping,
the water at the bottom continued to drop until it reached a temperature
of
34-35 degrees, at which time it stabilized. This was about the same
temperature
as the water just below the surface (about 4 inches). It never dropped
below
this temperature range except at the iar/water interface. When the
temperature
was just above freezing, both the surface and the bottom water stayed
around
34-36 degrees. The point is that unless a garden pond is dug well below
the
frost line, you are not likely to see much of the stratification you
normally
would see. This is why ponds that are not dug below the frost line can
and do
freeze solid during a cold winter.

## Gotcha! :-)

--
McKoi.... the frugal ponder...
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