What's the opposite of break up?

Freeze up? Well, I've shown plenty of pictures of break up (here, here, and here for example), but I've never really posted about the other end of the phenomena. I woke up bright and early on Friday and went for a walk down by the river so that I could see the progress of the ice on the river. The river is slowly freezing. There were chunks floating on the river making a clinking sound.
I had a nice walk and enjoyed watching the sun rise over the river around 9:30am. I stopped by the dock and watched a commercial fishing rig being pulled out of the water. Avery took a boat down to the village downriver for a wrestling tournament on Friday afternoon and had to take a plane home on Saturday afternoon because the river was even more frozen.

It's been fun to watch the river freeze since we put so much emphasis on watching it break up.