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' S CONTINENTAL DIVIDE 
BETWEEN TWO WATERSHEDS

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.