Wisconsin has more than 15,000 inland lakes covering about 4,000 km² (1 million acres), plus more than 800 km (500 miles) of shoreline on two Great Lakes, Superior to the north and Michigan to the east. The largest lake entirely inside the state is Lake Winnebago, at about 530 to 560 km² depending on the survey; the deepest natural lake is Green Lake, 72 m (237 ft) deep; and the largest artificial lake is Petenwell Lake on the Wisconsin River. Almost all of these lakes are glacial, which is why they crowd the north and east and nearly vanish in the unglaciated Driftless Area of the southwest. The state’s surface runs from Timms Hill, 595 m (1,951 ft), down to the Lake Michigan shore at 176 m (579 ft), and the geographer Lawrence Martin divided it into five regions. Each of those regions has the lakes its ice history gave it, from the Apostle Islands in the north to Lake Pepin on the Mississippi.
Lake Superior Lowland and the Apostle Islands
The northern edge of the state is a narrow, flat plain sloping gently down to Lake Superior, the largest freshwater lake in the world by surface area. Forests of aspen and birch cover much of it, and lake-effect storms make it Wisconsin’s snowbelt, with up to 4.1 m (160 in) of snow in a year. The region’s defining water is Superior itself, which Wisconsin shares with Minnesota, Michigan and Ontario.
Off the tip of the Bayfield Peninsula lie the 22 Apostle Islands, remnants of sandstone laid down by ancient rivers more than 500 million years ago. Water flowing beneath the continental ice sheet and, later, Superior’s storms cut the peninsula’s end into separate islands, and waves have hollowed sea caves into their red cliffs. All the islands except Madeline Island belong to the Apostle Islands National Lakeshore, where more than 800 plant species grow in a largely boreal forest.
Northern Highland: Vilas County’s 1,318 Lakes
South of the lowland the land rises into the Northern Highland, home to the state’s highest point and its most lake-studded country. Here the retreating Laurentide ice sheet left thousands of glacial lakes among mixed hardwood and conifer forest, the densest concentration in the state. Vilas County alone holds 1,318 lakes, and water covers 420 km², or 16 percent, of its area. The Wisconsin River, which gives the state its name, begins at Lac Vieux Desert on the county’s border with Michigan’s Upper Peninsula and runs about 690 km to the Mississippi.
Trout Lake, at about 1,560 ha (3,864 acres) and 36 m (117 ft) deep, is one of the largest natural lakes in the county. Farther west, the region’s biggest waters are reservoirs. The Winter Dam on the Chippewa River created Lake Chippewa, the Chippewa Flowage, whose 233 miles (375 km) of shoreline wind around forested islands, and the Turtle-Flambeau Flowage in Iron County covers about 52 km². The Chequamegon–Nicolet National Forest, about 6,100 km², spreads across much of the region, and Timms Hill in Price County, the state’s highest natural point, was only surveyed at its full 595 m in 1962.
Central Plain: Glacial Lake Wisconsin’s Reservoirs
The Central Plain is flat and sandy for a specific reason: it was a lake bed. Around 18,000 years ago the Green Bay Lobe of the ice sheet pushed against the Baraboo Hills and blocked the Wisconsin River, backing water up into Glacial Lake Wisconsin. The lake reached depths of about 49 m (160 ft) and a surface eight times that of modern Lake Winnebago, stretching north to the site of Wisconsin Rapids. Sandstone islands in it survive today as isolated bluffs such as Mill Bluff and Roche-a-Cri. When the ice retreated about 14,000 years ago, the lake probably drained within weeks or even days, and the torrent cut canyons through the soft Cambrian sandstone to form the Dells of the Wisconsin River.
The old lake bed is now home to the state’s largest artificial lakes. Petenwell Lake, formed in 1948 when the Wisconsin River Power Company dammed the river near Necedah, covers about 93 km² (23,040 acres), the second-largest lake wholly inside Wisconsin, with a maximum depth of only 13 m. Castle Rock Lake, the next reservoir downstream, takes Petenwell’s outflow and is mostly just 2.4 to 6.1 m deep, with only the drowned river channel reaching 11 m.
The Baraboo Range and Devil’s Lake
At the southern edge of the Central Plain, the Baraboo Range breaks the pattern. Geographers place it in the Western Upland, and it marks that region’s eastern boundary with the glaciated country. Its quartzite began as sediment in a shallow sea about 1.7 billion years ago, was later metamorphosed and folded into a syncline about 40 km long, and now stands in two ridges, the North Range and the South Range. Only the eastern end of the range was glaciated in the last ice age: Devil’s Lake State Park, the largest state park in Wisconsin at 3,730 ha, sits on the western edge of the Wisconsin glaciation.
Devil’s Lake fills a gap through the South Range, beneath quartzite bluffs 150 m (500 ft) high. The gap was once a gorge of the Wisconsin River; during the last glaciation a lobe of ice dropped terminal moraines across both ends of it, the north moraine about 24 m thick and the southeast moraine about 40 m, turning the gap into a 146 ha (360 acre) lake with no surface outlet. Its English name is a misinterpretation of the Ho-Chunk name Te Wakącąk, better translated as “Sacred Lake” or “Spirit Lake”.
Western Upland: Lake Pepin in the Driftless Area
West of Baraboo the rest of the Western Upland lies in the Driftless Area, which the last ice sheet never covered. Without glacial deposits there are no kettles and almost no natural lakes; instead, streams have cut deep valleys between ridges, and the region holds many of Wisconsin’s caves, including Cave of the Mounds. Even the state’s deepest lake of any kind, Wazee Lake near Black River Falls in Jackson County, is man-made: a taconite mine pit, worked from the mid-1960s until 1983, that flooded to about 108 m (355 ft).
The region’s great natural lake sits on its border. Lake Pepin, shared with Minnesota, is a widening of the Mississippi 35 km long and up to 3.2 km wide, covering about 100 km². It occupies a valley carved by Glacial River Warren, the outflow of Lake Agassiz, and it exists because the Chippewa River dumps so much sediment at its mouth near Wabasha that its delta dams the Mississippi. Over 10,000 years, sediment has pushed the lake’s upper end about 80 km downstream from near Saint Paul to below Red Wing. The villages of Pepin, Maiden Rock and Stockholm line the Wisconsin shore beneath the bluffs.
Eastern Ridges and Lowlands: the Winnebago Pool
The eastern third of the state was shaped by two tongues of ice, the Green Bay Lobe and the Lake Michigan Lobe. Where the western lobe scooped out its basin it left Green Bay, the Horicon Marsh and Lake Winnebago, a remnant of Glacial Lake Oshkosh about 12,000 years ago. Winnebago is about 48 km long and 16 km wide but averages only 4.7 m (15.5 ft) deep and nowhere exceeds 6.4 m.
Winnebago is the centre of a connected system called the Winnebago Pool. The Wolf and Fox rivers meet at Lake Butte des Morts, which links to Lakes Poygan and Winneconne upstream; the Fox then flows through Oshkosh into Winnebago and out at Neenah-Menasha toward Green Bay, as part of the Fox–Wisconsin Waterway. Oshkosh, Neenah, Menasha and Fond du Lac all sit on the shore, and municipal systems serving more than 200,000 people draw drinking water from the lake.
Madison’s Four Lakes, Green Lake and the Kettle Moraine
Where the two ice lobes collided, they piled up the Kettle Moraine, a belt of hills and kettle hollows running from Walworth County north to Kewaunee County. Farther west, the same ice sheet shaped the lakes of Madison. Ice more than 300 m thick covered Lakes Mendota, Monona, Waubesa and Kegonsa until it began retreating about 14,000 years ago; a single large lake then stretched from northern Mendota to Stoughton before falling water levels split it into four about 10,000 years ago. The state capital sits on the isthmus between Mendota, the largest at about 40 km², and Monona. Mendota’s present name dates only from 1849, when a surveyor named Frank Hudson proposed it; the Ho-Chunk had long called the lake Wąąkšikhomįkra, “Where the Person Rests”, and the four lakes still carry the Yahara River south through the city toward the Rock River.
Green Lake, in Green Lake County, is the deepest natural lake in the state at 72 m (237 ft), with an average depth of 30 m and an area of about 30 km², and it ranks second in volume behind Winnebago. Near the Illinois line, Geneva Lake reaches 41 m (135 ft). On the far eastern edge, the Door Peninsula divides Green Bay from Lake Michigan, its limestone Niagara Escarpment forming cliffs on the bay side and dunes on the lake side.
The regions line up with the ice: glaciated country is thick with lakes, and the Driftless Area has almost none. For the Great Lakes on either side, see our guides to Lake Superior and Lake Michigan, compare the same glacial pattern across the border in the lakes of Minnesota, or follow the state’s rivers and landforms further in our Wisconsin geography guide.