Monona Bay and Lake Monona are part of the Yarhara River chain of lakes. The lakes were formed about 10,000 years ago when the glaciers retreated. The Yahara watershed is about 359 square miles (over ¼ of Dane County). Much of the watershed is farmed; however, the watershed also contains most of the urban land in the Madison area. In addition, the Yahara watershed includes some of the largest wetlands that are left in Dane County.