Have you ever noticed that the water in a rapids or just below a dam rarely freezes even in the coldest weather? The reason why is simple: Water that is above freezing doesn’t freeze. Consider the low-head dam on the Red River in downtown Fargo-Moorhead. In cold weather, the river freezes over upstream from the dam, but remains open in the area immediately downstream. This open water has been flowing underneath the ice upstream where it was not exposed to the outside air, and so has remained warmer than the ice on top.
Once exposed to the cold air below the dam, it takes a while to cool down to freezing, which is why the top of the river freezes over again some distance downstream of the dam. In our coldest weather, this section of open water becomes much smaller. In a winter mild spell, it will lengthen considerably.