Lake Winnebago; Calumet, Fond du Lac, Outagamie, Winnebago Counties. Winter, 1970. Altitude c. 11,000 feet. Virtually all of the ice and snow covered surface of Wisconsin's largest inland body of water is seen to the northern horizon from a point south of the city of Fond du Lac. Almost thirty miles in length and over eleven miles in width, it attains a maximum depth of only about twenty-two feet. The lake is a remnant of the once-larger Glacial Lake Oshkosh.