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' S CONTINENTAL DIVIDE
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
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
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
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Update: June 2012