Alright, this is emphatically not a whitewater site. Lake Columbia (or Columbia Lake, I've seen it referenced each way) is a 500-acre cooling pond for a power plant near Portage.
The lake is approximately 1.8 miles long and up to a half-mile wide, with an average depth of only 7-8 feet. A dike extends from the powerplant (north end) most of the way down the center (suggesting there is likely some minor flow, down one side and up the other). It appears as though the east side is the outflow, and the west side is the intake (which appears to be significantly gated/shielded to avoid taking fish, etc, into the plant). It is unknownn (to this author) what sort of restrictions may exist regarding how close one can get to the outlet, and whether there are any currents or eddylines where that outflow meets the waters of the lake.
Water temperatures reportedly stay around 80 degrees (obviously varies with distance from power plant discharge and other factors), making it a popular place for paddlers during the winter months. Where else do you have an opportunity to practice your roll (or flatwater tricks) _outdoors, in warm water, in winter,_ in the _Upper Midwest_?
In the cold of winter, expect fog to cover much of the lake.
_GoogleMaps linked video_
Also, expect to share your day with many fishermen, as the lake is stocked by Wisconsin DNR.