Great Lakes

Seasonal Lake Stratification

Examine temperature profile data from the Great Lakes Environmental Research Lab and discover patterns in seasonal lake stratification.

Great Lakes Sometimes called Freshwater Seas, the Great Lakes form the largest fresh surface water system on Earth, and can easily be viewed from the moon. This system significantly impacts weather and climate, among other things, and therefore is not ignored by ocean scientists. The lakes - Superior, Michigan, Huron, Erie, and Ontario - cover more than 94,000 square miles and drain more than twice as much land. They cradle about one-fifth of the world's fresh surface water supply and nine-tenths of the U.S. supply. If the lakes' water were to be spread evenly across the contiguous 48 states, it would be about 9.5 feet deep!

The lakes are connected by rivers, which have long been transport channels for ships. Between Lake Erie and Lake Ontario, the 35-mile Niagara River sends approximately 50,000 to 100,000 cubic feet of water per second over Niagara Falls. From Lake Ontario, the water flows from the St. Lawrence River to the Atlantic Ocean, about 1,000 miles away.

Such a vast area of lakes and rivers is difficult to visualize, but perhaps these images will give you a feel for the grandeur of the region. Each lake is unique, but what the lakes do have in common is their interest to the scientific community, for many reasons. Issues of concern surrounding the Great Lakes include but are not limited to pollution, runoff, and the introduction of exotic species. The environmental and physical characteristics of the lakes are studied and documented in an attempt to use knowledge about the system for the purpose of preservation.

Data Activity
Great Lakes Using the Great Lakes Forecasting System database, we will examine seasonal changes in temperature stratification in Lake Erie. Temperature stratification also occurs in the ocean.

It is important to monitor lake temperatures so that scientists can be aware of any potential long-term climate changes. Lake temperature data is also useful for boaters and the shipping industry. In fact, these are the main users of the lake temperature data we will examine, and in this case, data recording stops in the ice season because these activities are not happening when the lake freezes over.

To understand why you are seeing such unique temperature profiles from season to season, you will need some background information on lake stratification. After reading up on this interesting natural phenomenon, answer the following questions:

  • What is the epilimnion?
  • What is the thermocline (or metalimnion)?
  • What is the hypolimnion?
  • At what times of year is a lake likely to be stratified or stagnant?
  • At what times of year is a lake likely to experience "turnovers", resulting in uniform lake temperatures?

During the spring warming period, the rapidly warming nearshore waters are inhibited from moving to the open lake by vertical thermal bars. These sharp temperature gradients, influenced by distance from shore, prevent mixing until the sun warms the open lake surface waters or until the waters are mixed by storms.

Let's explore some temperature data from the NOAA Great Lakes Coastal Forecasting System, Great Lakes Environmental Research Lab and the National Weather Service. Print out the following temperature profiles from the year 2006:
(Note: You may want to click on the image enlargement button in the bottom right corner)

April 10
May 10
June 9
July 9
August 8
November 11

Note on reading the temperature scales: There are 3 buoy readings included for each date. The depth is given on the y-axis and the temperature is given on the x-axis.

Compare and contrast the images.

  • What is the range of the water temperature in each image?
  • How can the stratification be described?
  • What is happening seasonally to cause these differences? Keep in mind that you may be seeing transitional changes.

Draw a simple diagram depicting a typical winter lake temperature profile.

The Bridge would like to extend a special thank you to Gregory Lang for making these data available for this activity.

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Read Me

Laura Rose, Virginia Sea Grant, Virginia Institute of Marine Science


Grade Level



Lesson Time

1 - 1.5 hrs.



  • Explain the process of seasonal lake stratification.
  • Compare and contrast temperature profile data to find stratification patterns.
  • Illustrate seasonal lake stratification processes.


Temperature stratification, Lake stratification, Epilimnion, Thermocline, Metalimnion, Hypolimnion, Temperature profile


Materials Required

temperature profiles (6):
April 10
May 10
June 9
July 9

August 8
November 11


Natl. Science Standards

IK-1 IK-2 PH5-3 ES5-1 ES9-1


Gregory Lang, NOAA Great Lakes Coastal Forecasting System, Great Lakes Environmental Research Lab and the National Weather Service


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