Great Lakes: Introduction

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Physical Characteristics

The Great Lakes – Superior, Michigan, Huron, Erie and Ontario – span more than 750 miles from west to east and are an important part of the physical and cultural heritage of North America.  PhotobucketThe complexity and magnitude of the Great Lakes water system is difficult to grasp, even for those who live within the basin.

The lakes cover a total of 294,000 square miles and contain about 5,500 cubic miles of water.  The Great Lakes are the largest collection of fresh, surface water on earth, containing roughly 18 percent of the world supply. Only the polar ice caps contain more fresh water.   Although part of a single system, each lake is different.  

PhotobucketLake Superior is the largest in volume. It is also the deepest and coldest. Superior could hold all the other Great Lakes plus three more Lake Eries.

“Superior has a retention time of 191 years because of its size. Retention time is a measure based on the volume of water in the lake and the mean rate of outflow.”  Because of a cool climate and poor soils, most of the Superior basin is forested, with little agriculture. The forests and sparse population result in relatively few pollutants entering Lake Superior, except through airborne transport.

PhotobucketLake Michigan, the second largest, is the only Great Lake entirely within the borders of the United States.

“The northern part is in the colder, less developed upper Great Lakes region. It is sparsely populated, except for the Fox River Valley, which drains into Green Bay. This bay has one of the most productive Great Lakes fisheries but receives the wastes from the world’s largest concentration of pulp and paper mills.  The more temperate southern basin of Lake Michigan is among the most urbanized areas in the Great Lakes system. It contains the Milwaukee and Chicago metropolitan areas. This region is home to about 8 million people or about one-fifth of the total population of the Great Lakes basin.”

PhotobucketLake Huron is the third largest of the lakes by volume.

“Many Canadians and Americans own cottages on the shallow, sandy beaches of Huron and along the rocky shores of Georgian Bay. The Saginaw River basin is intensively farmed and contains the Flint and Saginaw-Bay City metropolitan areas. Saginaw Bay, like Green Bay, contains a very productive fishery.”

PhotobucketLake Erie is the smallest of the lakes in volume and is most vulnerable to the effects from urbanization and agriculture.

“Because of the fertile soils surrounding the lake, the area is intensively farmed. The lake receives runoff from the agricultural area of southwestern Ontario and parts of Ohio, Indiana and Michigan. Seventeen metropolitan areas with populations over 50,000 are located within the Lake Erie basin. Although the area of the lake is about 10,000 square miles, the average depth is only about 62 feet. It is the shallowest of the five lakes and therefore warms rapidly in the spring and summer, and frequently freezes over in winter. It also has the shortest retention time of the lakes, 2.6 years. The western basin, comprising about one-fifth of the lake, is very shallow with an average depth of 24 feet and a maximum depth of 62 feet.”

PhotobucketLake Ontario, although slightly smaller in area, is much deeper than its upstream neighbor, Lake Erie, with an average depth of 283 feet and a retention time of about 6 years.  The Toronto skyline in visible in this picture.

Water Quality

Over 33 million people live in the Great Lakes basin.  We were much better stewards of our environment 50 years ago.  

In the late 1960s, growing public concern about the deterioration of water quality in the Great Lakes stimulated new investment in pollution research, especially the problems of eutrophication and DDT. Governments responded to the concern by controlling and regulating pollutant discharges and assisting with the construction of municipal sewage treatment works. This concern was formalized in the first Great Lakes Water Quality Agreement between Canada and the U.S. in 1972.

Major reductions were made in pollutant discharges in the 1970s. The results were visible. Nuisance conditions occurred less frequently as floating debris and oil slicks began to disappear. Dissolved oxygen levels improved, eliminating odor problems. Many beaches reopened as a result of improved sewage control, and algal mats disappeared as nutrient levels declined. The initiatives of the 1970s showed that improvements could be made and provided several important lessons beyond the cleanup of localized nuisance conditions.  


Toxic contaminants pose a threat not only to aquatic and wildlife species, but to human health as well, since humans are at the top of many food chains. Aquatic and wildlife species have been intensively studied, and adverse effects such as cross-bills and egg-shell thinning in birds, and tumors in fish are well documented. There is less certainty about the risk to human health of long-term exposure to low levels of toxic pollutants in the lakes, but there is no disagreement that the risk to human health will increase if toxic contaminants continue to accumulate in the Great Lakes ecosystem.

Long-term, low-level exposures are of concern because of subtle effects that toxic contaminants may have on reproduction, the immune system and development in children. Relationships between environmental contaminants and diseases such as cancer are also of concern.

Brrrrr ~~ Lake Michigan’s Grand Haven Lighthouse during a winter storm.


Note:  Pictures and facts courtesy of the US Environmental Protection Agency, or whatever is left of it.  They have published a very large study on the Great Lakes, and I hope to bring it to you in bite sizes pieces.    


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    • dkmich on September 28, 2008 at 19:55
    • kj on September 29, 2008 at 04:15

    dkmich.  i grew up south of the Great Lakes and we traveled north to one or another of the lakes every summer. they’re where i learned to swim and got caught and tumbled over by a tide.

  1. Great essay.

    I`ve been to all of the lakes, in summer/winter/spring & fall.

    They are really a sustaining force, sometimes adversely for man, but in the big picture a huge positive. Keeping these lakes (& all of the planet) clean, is a priority  for the future generations that will be a lot more dependent on fresh water than the bottled water wasters of today. It seems easier to be concerned & to follow up on that concern, when it comes to a lake, no matter the size. It is a contained unit with a lot of inflow variables, but controllable outflow points.

    Even with long retention times it is possible to measure differences in these outflow points, giving direction to where corrections need to be addressed.

    For example, a high sulphur reading might lead to stricter controls in the pulp & paper industry, whereas, phosphates/nitrates might point to lack of oversight in the agricultural field. (punny)

    The future depends on the education of us all,  which as an educator, you so well provide.

  2. I’m glad you posted this.  It adds to my education and empathy.

    • dkmich on September 29, 2008 at 13:16

    DD and the people here are “excellent”.  One day, I hope I have the pleasure and the privilege to meet you all.  

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