Why lakes are some lakes mucky, weedy and others sandy and clean

Why your lake is the way it is

Whether your lake bottom is mucky, sandy or rocky is determined by one simple factor: the age of your lake. Limnologists (scientists who study lakes) sort lakes into three main categories. Your lake is either young, middle-aged or old. The scientific names are:

Oligotrophic — Young lakes
Mesotrophic — Middle-aged lakes
Eutrophic — Old lakes

Young lakes have rocky bottoms. They’re deep and cold, the banks are steep and the shoreline vegetation is mostly pines and other conifers. Lake Superior is an example of an ‘oligotrophic’ lake.

Middle-age lakes have sandy bottoms with very few aquatic weeds. The banks have become more gentle with deciduous trees, like oak and maple, mixed with conifers. The sand bottom is the result of rocks grinding together for thousands of years. Lake Michigan is an example of a ‘mesotrophic’ lake.

Old lakes are full of nutrients that lake weeds thrive on. Every minute of every day, small particles of dirt, leaves, and other organic stuff blow into and wash into lakes, settling to the bottom. Over time, these particles build up as soft sediment — what we call “muck.” Your lake is a ‘eutrophic’ lake if it has a mucky, weedy bottom.

Some lakes age faster than others

The process of lakes aging is called “eutrophication.” A lake may have both sandy (mesotrophic) areas and mucky (eutrophic) areas. Such lakes are transitioning from middle-age to old-age.

“Hypertrophic” is a classification sometimes used for lakes nearing the end of their lifespans. We call them “swamps.” Their surfaces are covered with algae and floating plants, like duckweed and lily pads. The water is shallow, stained and murky. Filling up with sediment, they’re on their way to becoming “wetlands” and eventually, “good farmland.”

This is the natural aging progression of lakes and ponds. Human activity can speed up the aging process; agricultural runoff, lawn fertilizers, and leaking septic systems all add nutrients that lake weeds thrive on. But even lakes untouched by human activity will age, some faster than others.

The average lifetime of a lake is 10,000 years

The average lifespan of a lake is 10,000 years. Smaller lakes may live less than a thousand years, depending on how quickly sediments build up. (We own a 20-acre wetland that used to be a lake 2,000 years ago).

There are rare, “super agers,” — lakes over a million years old. The reasons they get so old are a bit more complex. Super agers are nearly all “rift lakes,” formed by plate tectonic activity. Most lakes we’re familiar with were formed by retreating glaciers.

We can’t stop the aging process, but we can slow it down by minimizing nutrients that fertilize lake weeds. We can use “lakescaping ” to inhibit runoff from yards and preserve shorelines.

You can also improve your enjoyment of your lake, whatever your bottom is like.