IOWA ACADEMY OP SCIENCES. 91 



removed by the Lake Agassiz outlet throughout the entire dis- 

 tance from St. Paul to Cairo. It is estimated that the average 

 width of the channel formed by this outlet is three miles, 

 or about four times the breadth of the present steam. 



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

 portions of the stream a lower level and lower gradient than 

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

 portion above the upper rapids, as indicated by General War- 

 ren.* 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 conse- 

 quence 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 Mississippi 

 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 

 necessary to cause the neighboring rapids to become protected 

 from erosion. It is not probable, however, that either of 

 these tributaries 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 advantage of much greater volume than 

 these tributaries, in consequence of which the fall across the 

 rapids must be reduced below that of the tributaries before 

 filling can begin at their mouths. 



CONTOURS OF THE BLUFFS ALONG THE LOWER 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 points, where the river in rounding a 

 curve has recently encroached upon its bluff, there is not an 



'Op. clt pp. 911-916. 



