538 THE ILLINOIS GLACIAL LOBE. 



LAKE MICHIGAN DRAINAGE BASIN. 



Lake Michigan receives the drainage of only a very narrow belt in 

 northeastern Illinois and northwestern Indiana, comprised mainly in the 

 drainage areas of Chicago and Calumet rivers. It drains about one-half 

 the area of the southern peninsula of Michigan and 1,500 square miles of 

 the northeast part of Indiana. It drains also an area of several thousand 

 square miles in the northern peninsula of Michigan and adjacent portions 

 of Wisconsin, mainly tributary to Green Bay. South of the Green Bay 

 drainage system only a narrow belt is tributary to the lake. The water- 

 shed draining to Lake Michigan is estimated to be 45,000 square miles, 

 and the total area of the basin 68,100 square miles, the lake area being 

 22,400 square miles. 1 



In the present discussion only that portion of the watershed is con- 

 sidered which borders the southern end of the Lake Michigan Basin and 

 lies within the limits of the Illinois glacial lobe. 



CHICAGO RIVER. 



With the exception of a few miles at the headwaters of the North 

 Fork, this small drainage system lies within the limits of Lake Chicago. 

 The South Fork, as previously noted, apparently' has afforded a line of 

 discharge for the Des Plaines River from the time of the withdrawal of the 

 lake down nearly to historic times, the size and depth of its channel being 

 such as would seem to demand the work of a stream as large as the Des 

 Plaines. The southward course of the North Fork, outside the limits of 

 the lake bottom, is occasioned by till ridges of the Lake Border morainic 

 system, the one on the east preventing direct discharge into Lake Michigan. 

 Within the limits of Lake Chicago the stream follows the slope of the old 

 lake bottom. 



CALUMET KIVER. 



The headwaters of the Calumet River are in the Valparaiso morainic 

 svstem south of Michigan City, Indiana, Its several southern tributaries 

 are also found in the inner slope of the \ r alparaiso morainic system. These 

 tributaries lead down the slope to the plain covered by Lake Chicago. 

 Their courses are there controlled to some extent by the line of sand dunes 



1 Rept, U. S. Deep Waterways Commission, 1896, p. 149. 



