What I have a hard time wrapping my mind around, is where is that heat in the summer? Is the running water really that much warmer than still water.
Recall that the earth below is an INFINITE heat source and sink, and runs pretty close to 50 degrees F ;-)
But to be "even more informative"......
Consider the formation of Water ice on the surface of any body of water. (let us exclude mechanical mixing just for this moment)
When water molecules cool, they move less, vibrate with a bit less amplitude then their warmer. more energetic brethren. and so take less space per molecule. As a group, when cooled, water becomes more dense, and so SINKS to the bottom of the "vessel" that contains it. River bed, Lake bed or ocean bed. All the same. All this sinking is CONVECTION.
As the water continues to cool, almost to the freezing temperature, a bit of magic occurrs. The arrangement of the two Hydrogen atoms SHIFT relative to the Oxygen atom, and the VERY COLD water suddenly get's BIGGER, Taking up more volume. The coldest water becomes LESS DENSE than the warmer water above it. This expanding process releases HEAT. The Heat of Transformation. So the critically cooled water floats UP to the surface. When two or more of these Very Cold , expanded water molecules come into contact, they can and will hook together in certain defined crystal structures. Remaining that way, and joined by others, SOLID water ice is formed. Ice is always formed from the waters that once were at the bottom most levels. The position change due to CONVECTION and a bit of black magic as pertains to heat energy exchange.
But in consideration of a flowing stream, or a river. the mechanical mixing, by the means of flow or tumbling over terrain etc., serves to keep the waters at differing temperatures from stratifying. Molecules that have given up their heat sufficiently to form ice, bump into water molecules that have an abundance of heat, enough to keep crystal ice from forming. It is entirely like a bunch of small children playing rough and tumble, long after they should be sleeping soundly. But note at the river's edges! The ice builds. Molecule by molecule . Some ice is heated and transforms again to liquid water, some critically cooled molecules collide and adhere, and so add to the solid ice.
The ice formed in the mist and sprays. The small floatsom of crystal clumpings, drifting at the surface, along with the eddy and flow of the stream. The clear ice sheets with feathered patterns growing in among the reeds. The small crystal collars around a protruding twig. Ever changing in size and structure. Now more, now less. Teasing the eye.
What a wonder.
And you might consider, IF water Ice did not have these characteristics. Lakes and streams would freeze from the bottom up. Never to thaw in temperate climates. Life, as we know, would be impossible.
It rains outside at this very moment. The land around had been an entire sheet of ice until the 50 degree air had melted things today. Still, the ground is frozen. the rain and melt will run to the frozen rivers. There will be flooding. The wood fire flickers through the glass front stove. Cheering away the damp and cold.
Oh Well.
