Why doesn't this river freeze?

Polar Bear Plunges, a tradition in many parts of the world, consist of people in regular bathing suits (and sometimes without) taking a communal swim in a lake or river in winter. While there is a psychological question about why people might do this, there’s also a physics question: Why is the water in the river above not frozen even though the air is below zero degrees Celsius?

1) There’s warmer water underneath that diffuses upwards to the cold water on top.
2) Since river water moves, the cold surface water will mix with warmer water from below the surface, heating it up and preventing it from freezing.
3) Both 1 and 2.
4) None of the above

Photo credit: Glenn Euloth

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