A couple days ago I went out to check on the pond and found the water was no longer running. That means everything was frozen. So yesterday I went out with some hot water and melted a hole in the ice to get to the pump. What apparently happened is the ice in the waterfall caused the water to overflow causing the pond to drain to the level of the pump. So luckily there was a space of about a foot between the water and the top of the ice.
Since the waterfall and hose to it are now frozen solid, I used a pruner to cut the hose and rigged up the diverter below to keep the water from shooting straight up in the air (which my gloves and coat can verify would happen), then lowered the pump a tiny bit. Checked on it today and all is well.
I think the snowstorm tomorrow will bring the water level up quite a bit so I can just top it off in the spring.