504 THE ILLINOIS GLACIAL LOBE. 



By this interpretation the Dupage and other tributaries of the lower Des 

 Plaines, as well as the portion of the Des Plaines itself outside the Valpa- 

 raiso morainic system, should be referred to a distinct drainage basin from 

 that of the upper Des Plaines. The upper Des Plaines, as above defined, 

 has an area of 524 square miles and a length of 62 miles (Cooley). If Salt 

 Creek be included, the area is 634 square miles. At the soiu'ce of the Des 

 Plaines is a slough which drains northward to Root River, a tributary of 

 Lake Michigan entering at Racine, Wisconsin, as well as southward to the 

 Des Plaines, and which stands 112 feet above Lake Michigan (Cooley). 

 The course of the upper Des Plain'es is governed by till ridges on its east 

 border, as indicated above, except for a few miles near its entrance into the 

 Chicago Outlet. It there traverses a southward-sloping plain. 



The course of Salt Creek also is governed by a till ridge on its east 

 border as far south as Fullersburg. It there turns east tln-ough a gap in 

 the ridge, but its old course was southward past Western Springs, through 

 Flag Creek Valley, to the Chicago Outlet near Willow Springs. Its old 

 valley is larger than the new one, from which it is inferred that the deflec- 

 tion is somewhat recent. The cause for the deflection is perhaps a silting 

 up at the mouth which followed the abandonment of the lake outlet. Con- 

 ditions for such silting are favorable, for there is scarcely any fall along the 

 bed of the outlet for several miles below the old mouth of the creek. This 

 interpretation, however, is one which has not been fully tested by observa- 

 tions in the field. 



For 30 miles below the point where the Des Plaines enters the Chicago 

 Outlet it traverses the Valparaiso morainic system and receives no tributaries 

 of importance, for much of the drainage there is only the bluff drainage. 

 It has not in this interval perceptibly modified the bed of the old lake out- 

 let. There is the "12-mile level" below Summit, soon followed by the 

 descent of about 70 feet in 8 miles. Farther down are two pools — one below 

 Joliet, known as. Lake Joliet, another near the mouth of Dupage River, 

 known as Lake Dupage — which are separated by an interval of 3 miles of 

 slope with a fall of about 13 feet. There is also a fall of about 2£ feet in 

 the half mile from Lake Dupage to the junction with the Kankakee at the 

 head of the Illinois. The only true flood-plain bottoms are in the 7 miles 

 between Lake Joliet and the head of the Illinois. These lie within the 

 range of backwater from the Kankakee and are overflowed only in case of 



