THE CHICAGO OUTLET. 425 



ciently strong to carry away nearly all the detritus brought into it by the 

 side streams 



The rapids between Komeo and Joliet occur in a section where the 

 limestone is friable, and it is thought by Professor Cooley that the friability 

 is such that falls could not have been maintained, or even established. The 

 removal of the existing rapids, the main barrier in the course of the outlet, 

 it is estimated, would require the excavation of a channel in rock only 

 about 20 miles in length and 25 to 75 feet in depth. This excavation 

 would be about ten times that accomplished by the lake outlet in that part 

 of its course. Being the outlet from a lake, the amount of sediment carried 

 by its waters is a matter which should be weighed in discussing the slight 

 amount of excavation. 



Professor Cooley has called the writer's attention to the deposits at the 

 head of Lake St. Clair as likely to furnish an index of the amount of sedi- 

 ment transported by the Chicago Outlet. A delta with an area of several 

 square miles has been built in the head of Lake St. Clair, which must have 

 derived the bulk of its material from southward-moving littoral currents 

 along both the borders of Lake Huron. In the lake under discussion littoral 

 currents along the west border would have transported material probabty in 

 as great volume as on either shore of Lake Huron, but those on the east and 

 south may have contributed less, for wind drifting there is very effective. 

 It seems legitimate to assume that at least half as much sediment was being 

 transported down the Chicago Outlet as is carried by the St. Clair River. 

 From this it appears probable that the waters of the Chicago Outlet were 

 somewhat less turbid than the St. Clair. Professor Cooley thinks the 

 contributions of sediment to the outlet through the Des Plaines were of 

 little consequence, for this river has, since the lake waters were withdrawn, 

 made scarcely any filling of the outlet below Riverside, where its delta 

 would naturally accumulate. The accession of larger tributaries below 

 may have rendered the stream slightly more turbid than on the rapids 



It should not be inferred that this outlet is entirely free from river 

 debris. Beginning at the upper beach, near Summit, there is for several 

 miles a mass of coarse material, largely limestone blocks, too large to have 

 been transported by the current, covering the bed of the outlet. The 

 Drainage Canal exposes excellent sections of the coarse river debris from 

 Summit to Lemont, there being only limited areas in this interval where 



