Lakes, Rivers, and Wetlands

A state made of water

Wisconsin has more than 15,000 inland lakes, 84,000 miles of rivers and streams, and about 800 miles of Great Lakes shoreline ¹. Most of the lakes are in the glaciated north, where the ice left kettles, the hollows of buried ice blocks, and moraines that dam water. The Driftless southwest, never glaciated, has almost no natural lakes at all; its water runs in streams.

Where a lake’s water comes from

Lakes are sorted by their plumbing: whether they have an inlet, an outlet, and groundwater ².

• Seepage lakes: no inlet or outlet. Their water comes from rain, runoff, and groundwater from right around them, so their levels rise and fall with the weather. They’re the most common type in Wisconsin, and with small drainage areas they’re often low in nutrients, with clear water and simpler fisheries ².
• Spring lakes: no inlet, but an outlet. Groundwater flows in through the bottom. They’re the headwaters of many streams and fairly common in the north ².
• Drained lakes: an outlet but no inlet and no groundwater, fed by rain and runoff; the least common type ².
• Drainage lakes: an inlet and an outlet, fed mainly by a stream, and usually richer in nutrients than seepage or spring lakes; most major rivers have them along their course ².
• Impoundments: a lake where a dam makes half or more of the maximum depth ², like the flowages of the north and the millponds of the south.

The type predicts a lot: a nutrient-poor seepage lake in Vilas County and a stream-fed drainage lake in farm country can sit under the same sky and look nothing alike.

Layers and turnover

A deep Wisconsin lake doesn’t stay mixed. Water is densest just above freezing, at about 39 °F. In summer, the sun warms the surface, and that warm, light water floats on a cold, dense bottom layer, with a sharp temperature change between them, the thermocline. The bottom layer is cut off from the air for months, and its oxygen slowly runs down. In fall, the surface cools until it matches the water below, and wind mixes the whole lake top to bottom: fall turnover. Under winter ice the layers reverse, with the coldest water at the top, and in spring, after ice-out, the lake mixes again. Oxygen goes back to the bottom, nutrients come up, and fishing patterns change with it.

Rivers

Wisconsin has more than 12,600 rivers and streams across those 84,000 miles; about 32,000 miles flow all year, and the rest run only in spring and wet times ³. There are nearly 2,700 trout streams, more than 10,370 miles of them ³, most of them cold, spring-fed streams in the Driftless coulees and the north. The Mississippi runs about 200 miles along the state’s western edge ³.

Rivers here have been heavily dammed: about 4,700 dams, more than half privately owned ³. Taking dams out restores rivers. After four dams were removed in the 1990s, the Baraboo River became a 112-mile free-flowing stretch, one of the longest in the nation ³.

Wetlands

Wetlands are the marshes, sedge meadows, bogs, swamps, and fens between dry land and open water. They hold floodwater, filter runoff, and feed groundwater, and they’re some of the richest wildlife habitat there is.

Wisconsin once had about 10 million acres of them, judging by the extent of wet soils. The state’s wetland inventory, mapped from 1978–79 aerial photos, found about 5.3 million acres left, a loss of about 47 percent ⁴, mostly drained and tiled for farming or filled for building, and heaviest in the south. Horicon Marsh in Dodge County, the largest freshwater cattail marsh in the country, survived only after a failed attempt to drain it for farming.

What to take from this

Wisconsin has more than 15,000 lakes, 84,000 miles of rivers, and about 800 miles of Great Lakes shore. Lakes are sorted by plumbing: seepage (most common, no inlet or outlet), spring (groundwater, headwaters), drained (least common), drainage (stream-fed, richer), and impoundments. Deep lakes layer in summer and turn over in fall and spring. Nearly 2,700 trout streams run cold from springs, dams are coming out, and about 47 percent of the state’s wetlands are gone.