Forest Preserve District of Cook County, Illinois
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Chicago's Continental Divide
Nature Bulletin No. 571   September 12, 1959
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist 

Chicago, a little frontier settlement only 150 years ago, has become one of the world's greatest cities and the crossroads of America. It is the hub of transportation routes by highway, railroad, air and water. Its strategic location at the south end of Lake Michigan has been largely responsible. Among other unique advantages, it has an inexhaustible supply of soft fresh water.

Equally important is the fact that, paralleling the lake shore and not far from it, there is a continental divide: a ridge which, although low and inconspicuous, separates two vast watersheds. The term "watershed" is commonly applied to the drainage area contributing to the supply of a stream or a lake; also, as in this case, to a large number of watersheds all discharging, finally, through one great outlet.

On one side of this continental divide, here, the surplus rain and snow waters naturally drain toward Lake Michigan. This surplus, combined with that from the watersheds of the other Great Lakes, eventually flows into the St. Lawrence river which empties into the Atlantic Ocean. On the other side of this divide, water drains toward the Des Plaines and Kankakee rivers and thence down the Illinois river to the Mississippi which empties into the Gulf of Mexico.

The lowness of that divide facilitated the construction of railroads and highways, in all directions from Chicago, with the minimum of difficulty and expense. Southwest, near the village of Summit, it is only 15 feet above the surface of Lake Michigan and this is known as the Chicago Outlet -- one of the greatest natural passes in America the gateway to the Illinois valley.

Through this outlet, during extreme floods, the Des Plaines used to overflow eastward through Mud Lake and thence into the South Branch of the Chicago river. That was the route of the Chicago Portage traveled by early explorers, missionaries and traders. It facilitated the building of the Illinois and Michigan Canal which contributed so much toward the rapid early growth of Chicago. Later, to obtain a downgrade route southwestward, two great railroads were built through the outlet. The same route was naturally selected for the Chicago Sanitary and Ship Canal.

This canal, begun in 1892 and completed in 1900, is now a vital link in the most important system of inland waterways in the nation. Equally important is the part it plays in the disposal of the domestic sewage, industrial wastes and storm water from the Chicago metropolitan area. Prior to 1900, many of the combined sewers discharged directly into Lake Michigan; others discharged into the Chicago and Calumet river systems which emptied into the lake. The city's water supply became grossly polluted and the death rate from typhoid and other water-borne diseases was the highest in the country.

The main channel of the canal reversed the flow of the South Branch of the Chicago river and diverted it into the Mississippi valley. Through a navigation lock and control dates at the mouth of the Chicago river, it is flushed with fresh water from Lake Michigan. The north shore channel, through gates at Wilmette, diverts fresh water into the North Branch. The Calumet-Sag channel, completed in 1922, reversed the flow of the Calumet and Little Calumet Rivers and discharges it into the main channel.

This tremendous feat -- making the rivers "run backwards" was accomplished by the Sanitary District of Chicago. Together with the intercepting sewers and sewage treatment plants, it has been termed "one of the seven wonders of the modern world. " This was feasible only because of the low continental divide between the watersheds of the Chicago and the Calumet river systems and that of the DesPlaines river.

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