472 THE ILLINOIS GLACIAL LOBE. 



rapids (discussed on a preceding page), and following this filling a nearly 

 complete removal of it along the rapids. 



(3) The Iowan loess filling, also of undetermined amount, but probably 

 15 feet or more, and following this a nearly complete removal along the 

 rapids. 



(4) The Wisconsin sand filling, which seems to have raised the river 

 bed about 50 feet, followed by a nearly complete removal along the rapids. 



It would be very difficult to estimate the work involved in filling, even 

 though the depth of filling in the vicinity of the rapids were known. The 

 filling is simply an index of the excess of the material brought in over the 

 transporting power of the stream. To properly estimate the work done in 

 a stage of filling, it would be necessary to compute the amount of material 

 carried through the channel as well as that deposited in it. In the case of 

 the lower rapids, it seems doubtful if such computations can be made. 



The uncertainties involved in changes of drainage area are fully as 

 great as in the work of filling. Some data showing the effect of- a change 

 of volume upon the gradient of the river are cited by General Warren. 

 At the time when Lake Agassiz discharged through the Mississippi the 

 stream appears to have opened its channel in the Upper Mississippi to a 

 depth in harmony with the bed of Lake Pepin, which is about 60 feet 

 below present low water. The lesser volume of water now passing down 

 the Mississippi has proved inadequate to remove the detrital dam built at 

 the mouth of the Chippewa River (which is at the lower end of Lake 

 Pepin), and thus the bed has been raised in that vicinity about 50 feet. It 

 is thought by General Warren that a filling is now in progress along the 

 greater part of the Mississippi above Cairo as well as below, and that the 

 rapids are the chief places where a marked cutting is now in progress. 1 



In view of all these qualifying conditions, it seems hazardous to venture 

 a comparison of the work of the Mississippi at the lower rapids with that of 

 the streams on which estimates have been made, or at least to base definite 

 conclusions upon such a comparison. 



In closing this discussion attention is called to the contours of the bluffs 

 of the channel along the rapids and to the rate of fall in the rapids. The 

 contours of the bluffs favor the view that excavation began soon after the 



1 Bridging of the Mississippi : Ann. Rept. Chief of Engineers, U. S. Army, 1878-79, pp. 912-917. 



