The river environment and substrate can play a big role, too. Ever notice that ice typically forms first around rocks, logs or other solid objects the break the water surface, and around the shore?
Different environments make a big difference in ice formation and ice accumulation, too. Shallow and turbulent water (riffles and rapids) promotes the formation of "frazil" ice--small floating ice crystals. These do not normally impede the water flow, but when they hit something solid--a rock, a log, the shore, or a cap of ice over a less turbulent part of the river--they will stick to it, extending the ice cap. In places where it gets cold enough, shallow rivers can form "anchor ice", where conduction through the ground gets the stream bottom colder than the freezing point. At that point ice may start to freeze to the bottom and stick.
"Seed crystals" play a role, too. Water can be cooled to or even past the freezing point with no ice formation. Drop an ice crystal into the supercooled liquid, and the water may freeze up fast. For a cool science fiction take on this process (based on real science, stretched just a bit) check out Kurt Vonnegut's "Cat's Cradle".
And don't forget the relationship of pressure. Ever open a soda or beer, only to have it freeze as soon as the cap is off and the pressure from CO2 released?
Different environments make a big difference in ice formation and ice accumulation, too. Shallow and turbulent water (riffles and rapids) promotes the formation of "frazil" ice--small floating ice crystals. These do not normally impede the water flow, but when they hit something solid--a rock, a log, the shore, or a cap of ice over a less turbulent part of the river--they will stick to it, extending the ice cap. In places where it gets cold enough, shallow rivers can form "anchor ice", where conduction through the ground gets the stream bottom colder than the freezing point. At that point ice may start to freeze to the bottom and stick.
"Seed crystals" play a role, too. Water can be cooled to or even past the freezing point with no ice formation. Drop an ice crystal into the supercooled liquid, and the water may freeze up fast. For a cool science fiction take on this process (based on real science, stretched just a bit) check out Kurt Vonnegut's "Cat's Cradle".
And don't forget the relationship of pressure. Ever open a soda or beer, only to have it freeze as soon as the cap is off and the pressure from CO2 released?