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 OP 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. cit. pp. 911-916. 
