In reply to Alex Pryor:
I went up in late December about 10 years ago with a mixed group including non climbers when it was cold; the tarn was frozen over but the scar was flowing - not in spate. It was icy in places. Everyone made it up unroped, including someone with a large dog on their shoulders. It wasn’t for those of a nervous disposition mind you. I wouldn’t have wanted to descent it...
My abiding memory is having lunch out on the ice over the tarn, and that there were frozen vertical columns of bubbles frozen through the thick ice following the edges of the rocks on the bottom.