It looks like the earliest that they take water temp readings from the Trout Run is in April.
Source:
https://data.upperiowariver.org/sites/16-tru-10I used the: Monthly Water Sampling Data for Trout Run.
So basically the water that flows in Trout Run is the water that flows through the Hatchery
system including the Retention Pond.
The lowest temp I see was on 04/11/17 @
41.4° F. That is not going to freeze.
What the Trout Run water temp gets down to in Jan, Feb and Mar I don't know, but I suspect it remains above freezing in those months also.
In all the years of watching the Decorah Eagles (since 2011) I don't ever recall seeing the pond freeze over.
It's great that the Retention Pond stays unfrozen at this time of year for the local and passing through water fowl especially
considering how cold the air temps have been around there. 2nd Image below.
I suspect that the pond is full of all sorts of icky yummy things for the Geese.