20 FRANK L EVE RETT 



formed by this outlet is three miles, or about four times the 

 breadth of the present stream. 



The depth of erosion seems to have been such as to give 

 portions of the stream a lower level and a lower gradient than 

 that of the present river. This is especially noticeable in the 

 portion above the upper rapids, as indicated by General Warren. 1 

 Lake Pepin, an expansion of the Mississippi, situated just above 

 the mouth of the Chippewa River has a depth of about sixty 

 feet. It was General Warren's opinion that when the flow of 

 water from the great northern basin ceased, there would no longer 

 be the volume of water necessary to remove the deposits brought 

 in by the Chippewa River. In consequence of this change the 

 Mississippi has been lifted to a level about sixty feet above its 

 former bed. Evidence of a similar filling, produced by the 

 Mississippi at the mouth of the Minnesota, is cited by General 

 Warren. He also noted evidence of the marked shoaling of the 

 Mississippi at the mouth of the Wisconsin. He further expressed 

 the opinion that the entire cutting now in progress on the Mis- 

 sissippi may be confined to short sections in the vicinity of the 

 rapids. 



It is of interest to note what a slight change is required to 

 stop the cutting at these places. A filling of only twenty-five 

 feet at the mouth of the Des Moines, or of Rock River, is nec- 

 essary to cause the neighboring rapids to become protected from 

 erosion. It is not probable, however, that either of these tribu- 

 taries will for some time, begin the filling of the valley at the 

 foot of the rapids, for the fall of the Mississippi, in passing each 

 of the rapids, is greater than that of the lower course of the Rock, 

 or the Des Moines. Furthermore the main stream has the advan- 

 tage of much greater volume than these tributaries, in conse- 

 quence of which the fall across the rapids must be reduced below 

 that of the tributaries before filling can begin at their mouths. 



Cojitours of the bluffs along thejower rapids. — The great length 

 of time involved in the development of a channel across the 

 rapids is shown by the contours of the bluffs. Except at a few 



J Op. cit., pp. 911-916. 



