The reason that moving water usually doesn't freeze is that there is
usually some source of heat, the ground, the pump or something else,
and the circulation spreads the heat around so that it is less likely
for the water to freeze.


I see an illustration of this every winter. I live near Lake Winnebego,
in Wisconsin. Lake Winnebego is a widening in the Fox River. The lake is
about 10 miles wide and about 30 miles long. And it's shallow, most of
the lake is only about 10-15 feet deep. So there's lots of surface area
to cool the water from above and warm the water from the ground.

Nearly every December the harbor, where the water exits the lake over a
dam, freezes. What happens is that ice that forms upstream accumulates
behind the dam. At that point, usually mid December, the lake is still
largely open water.

Within a few weeks, by early January, the lake is usually completely
frozen over. But the ice clears from the harbor! What happens, I
believe, is that once the lake is frozen over, (a) there are no longer
any loose chunks of ice to float down stream, and (b) the only path for
the water flowing through the lake is along the bottom, where the water
is about 39 deg F. The relatively warm water upwells at the harbor and
melts any stray ice.

It is not unusual on a cold January morning - temperature below about 0
deg F - for clouds of steam to rise where the "warm" water flows over the
dam.


- Rich