I guessed (without having seen one before) that it might be a dipper and when we came home I looked on the net and found a great video from
Kids National Geographic (you have to watch about 30 seconds of animation before the film of the dipper comes on - but it is so great because they filmed the bird underwater and they explain how it can swim in such cold water.
It bobbed like a sandpiper or plover while it was on the ice, then it plunged into the water and stayed submerged for a considerable time, popped back up on the ice and then did the whole thing all over again. I stood transfixed for at least 15 minutes until it flew to another part of the river, Santiago dog was bored but patient. I have to say Charley was the one who spotted the bird on the ice in the first place....so kudos to him.
The dipper was a bit bigger than a junco, with taller legs of course and had a very short tail and was the grey slate colour of a junco. Try to watch the video if you can.
try http://video.nationalgeographic.com/video/player/kids/animals/bird-dipper