Age of Lakes

Bodies of water, like all living things, go through an aging process. In lakes, this aging process is known as “eutrophication,” it means aging.

Young lakes are called “Oligotrophic.” Traits of young lakes are:

  • Steep shorelines going down to the water’s edge.
  • Primarily conifer trees (pines) along the shore.
  • They are deep and drop off quickly.
  • The bottom is mostly rocky.
  • The water is very clear.
  • They have very few aquatic weeds.
  • They have high oxygen concentration.
  • They are populated with cold water fish such as trout, steelhead, whitefish and salmon.
  • Example: Lake Superior

Over several thousands of years, shorelines of oligotrophic lakes erode and become less steep. The rocks on the lake bottom grind against each other creating sand, while more plant life emerges on the shore and in the water. The lake reaches middle age.

Middle-aged lakes are called “Mesotrophic.” Traits include:

  • Less steep shorelines.
  • A mixture of conifer and deciduous trees, such as oak, maple and ash.
  • The bottom is mostly sand, resulting from erosion and weathering of rocks.
  • There are a few aquatic weeds and more plants growing on the shoreline.
  • They are less deep on average, than oligotrophic lakes.
  • The water is still very clear.
  • They support both cold and warmer water fish, such as bass, perch and bluegill.
  • Example: Lake Michigan

After several thousand more years, mesotrophic lakes gain nutrients from the fallen, decayed leaves from deciduous trees, decayed shoreline plants, decaying aquatic plants, and the remains of fish and other water creatures. The lake bottom near the shore becomes covered with silt and sediment. The lake has become an old lake.

Old lakes are called “Eutrophic.” Traits include:

  • Gentle, mostly flat shorelines.
  • Mostly deciduous trees along the shore and an abundance of shoreline plants.
  • Are usually quite shallow compared with sandy or rocky bottom lakes.
  • Very little oxygen in waters deeper than 30 feet.
  • The water is murky or “stained” from organic material and single cell planktonic algae.
  • Heavy aquatic weed growth.
  • Few, if any cold water fish. But bass, panfish, pike and carp thrive in old lakes.
  • Example: Probably your inland lake, or you wouldn’t be reading this.

It is the nature of all lakes to fill themselves in and become land. After several thousand more years, your lake will continue become shallower in the center, more shoreline will erode into the water, trees will fall in, leaves, dust and dirt will blow in, weeds will become thicker and grow out farther into the lake, die, decay and add to the bottom.

We have several names for bodies that were once lakes. They’re called bogs, swamps, wetlands and finally, “darn good farmland!” You won’t have to worry about this in your lifetime though. A very rough rule of thumb is a lake will fill in about two feet every thousand years. So if your lake is only 20 feet deep, it will be (on average) about 10,000 years before it’s filled in completely. Until then, enjoy it!


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