Central Wisconsin Essentials

The Basics of Geology and Natural Landscapes

Sunset Lake

Sunset Lake, the second deepest lake in Portage County, is a kettle lake. Kettle lakes are formed when enormous glaciers begin to melt and crumble. Large chunks of ice tumble off and are then buried beneath sand and gravel debris. As the buried ice melts, it leaves behind deep depressions, or kettles. This kettle lake is home to bluegill, crappie, and large mouth bass.

kettle lake diagram Kettle Lake diagram 

Blocks of ice from glaciers that were present 10,000 years ago melt and form kettle lakes.

Learn More: National Snow and Ice Data Cente

Minister Lake

Minister Lake averages about 16 square acres and 9 feet in depth. It is home to beavers, waterfowl, amphibians, and many macroinvertebrates. The water levels in Minister Lake fluctuate from year to year.
Minister Lake changes 

Seepage Lakes

Both Minister and Sunset Lake are seepage lakes that are filled with precipitation, runoff, and groundwater. They are then drained by groundwater.

seepage lakes diagram


Sunset Lake and Turnover

As water temperatures in Sunset Lake begin to drop during fall, some drastic changes occur. These changes all have to do with water density. Water has the strange property of being the least dense (lightest) when it is frozen. This is why ice floats in our drinks. Any warmer than 32 degrees and water becomes more dense (heavier) until it hits 39 degrees. At 39 degrees water is as dense as it can be, any warmer or cooler and it becomes less dense. This plays an important role in our lakes and creates a process called turnover.

What is Turnover?

Turnover occurs in lakes in both fall and spring, and is basically the upper and lower layers of water in a lake mixing together. In summer, Sunset Lake is organized by bands of water that are seperated by temperature. This is called thermal stratification. The top band is called the epilimnion and is a warm layer often moved by wind. The water in this band is all the same temperature. The middle band is called the thermocline and holds water whose temperature varies with depth. The thermocline is an invisible wall which keeps the bands of water above and below it from mixing. The bottom band is called the hypolimnion and holds cool, stagnant water which is all the same temperature. Sunset Lake seperates into these bands because its water is different temperatures and therefore different densities.


 water temperature bands diagram

In fall, the epilimnion (top band) cools and its temperatures become like those of the hypolimnion (bottom band). Once the two bands of water have similar temperatures and densities, the thermocline (middle band) disappears and wind is able to mix all the water together. This mixing is important because it adds oxygen to the deeper water, in which many creatures will need to spend the winter.

In winter, the water seperates into layers again, this time with cooler water in the epilimnion (top layer) and warmer water in the hypolimnion (bottom layer). When the ice melts in spring and warms up to 39 degrees, the water will again be able to mix and spring turnover will occur.


 The dark blue lines in the diagram below represent the thermocline.

 water temperature diagram


Learn more: Water on the Web


Nutrient Richness

There are three main levels of nutrient richness in lakes:
  • Eutrophic lakes--Have many aquatic plants and algal blooms which can result in winter kills.
  • Mesotrophic lakes--Have some vegetation and occasional algal blooms.
  • Oligotrophic lakes--Are very deep and have little vegetation or algal blooms.
A lake's trophic level can be determined in part by its phosphorus levels. Phosphorus is a key nutrient which determines how much plant and algal growth occurs. Excess phosphorus can enter a lake through septic systems, detergents, animal waste, farmland and storm sewer runoff, soil erosion, and fertilizers. In 2003, Sunset Lake had 25.5 parts per billion phosphorus, making it a mesotrophic lake.
Learn more: Water on the Web
(All images were created by Jim Buchholz)