Here is a photo of North Twin Lake in Yellowstone. Even though there was still a bit of snow and ice on the shore most of the lake was open. The South Twin Lake, on the other hand, was almost completely frozen. On the day that we arrived there was only a small bit of open water on the southern twin. However by the time that we left, five days later, the warm temps had melted a lot of the ice opening up about a third of the lake.