There are many things to love about winter scenery, and one of my favorites is the fast-flowing stream. On a clear day, the sun and the snow are brilliant in comparison to the darkness of the hemlock glen. The stream is both light and dark. There is a sensory buffet of pleasant stimuli from the sparkle of spray, the ice beside the water, the sound of the creek tumbling over the rocks. Contrast this with the quiet winter woodland in general, and sense of solitude in the austerity of cold times, and you get a remarkable hiking experience. The ice sculptures are worthy of attention, and often they are splendid in form. Some can only be found beside streams, and never at a frozen lake.
Streams differ from lakes by their energy, casting out droplets, tumbling downhill. The way energy is stored in streams and lakes differs greatly, too. The thermal capacity (storing heat) of water keeps lakes warm and streams cold. When the atmosphere is well below freezing (32F or 0C), lakes and streams behave very differently. Because ice is lighter than water, it floats. A lake will be covered in ice while the warmer water below is not frozen. Very cold air in a hard winter will pull heat out of a lake rapidly until the surface is freezing, at which point ice forms. Below, the water is still liquid, and as the winter wears on the ice will grow deeper and deeper, but the water in winter is warmer below in a lake.
Streams freeze very differently from lakes. The turbulence in the stream means that there is no upper cold layer that will freeze and a lower warmer layer that will not. The whole stream will be the same temperature, and so the whole stream must get down to freezing or the water will not freeze anywhere. Because there is a great deal of heat saved in water, it takes a long time to cool an entire watercourse to below freezing. Even when it has been frigid enough to freeze over the top inch of all the lakes, the streams will likely still flow because they are not entirely below freezing yet.
Because the whole stream is so very cold, drops of liquid water will freeze, perhaps instantly, once free of the stream and up in the colder atmosphere. Bits of ice form when ripples push water above the general level of the stream on rocks or other structures. Ice shelves begin to form just above the general level of the water, anchored to the rocks. Every ripple that wets the ice from below will add to it. If the stream is stable generally, and ripples repeat regularly, the ice may grow in a very orderly, crystal-like fashion because of this steady, slow addition, one tiny bit at a time. These ice sheets may become strong enough to support not just their own weight, but also a snowfall. They may also close the gap together over the water below, dividing the stream from the atmosphere above.
When the stream is close to freezing and there are significant blocks of ice in the stream, a sudden rise in the water level will put the stream above the ice. The water is too cold to melt the ice on a large scale, so the ice is essentially polished by the water. This creates a most remarkable sight, with the ice appearing rather blue when it is thick, and imparting that color to the water nearby. This is a unique winter sight, and one where the sensory impact is not preserved in a photograph. You need to see it for yourself.
If luck provides a sudden week of bitter cold, the ice shelves growing out from nearby surfaces will connect above the stream, and we may find a waterfall that is wearing a mantle of ice. Often, if you do not know what to look for, these are lumpy and unremarkable. At other times, they are lacy, delicate, translucent, and ethereal. These pretty forms can also be found on discharge pipes under highways and other such places where water passes out at velocity into the cold air. The forms can be very beautiful at a pipe beside the road, although the surroundings are much better in the woods. Look for where water flows in the coldest of winter, and you may find something super cool.