From the City of Madison website.
Let's go back in time to the formation of the pond. Ten thousand years ago a glacier buried a "chunk" of ice in the earth. When the ice later melted, a "kettle" formed in the void created and filled with water. Historically water levels in the pond would vary from year to year, depending on surface runoff from rainfall or spring snowmelt. Emergent plant cover would increase when water levels were low and decrease when water levels increased. Old air photos show that with low water levels the pond looked more like a marsh, covered with emergent plants and allowing only a small amount of open water in the center.