90 
IOWA ACADEMY OP SCIENCES. 
It scarcely needs to be stated that so great a filling has 
greatly inter rap ted the removal of the rock barriers of the 
Mississippi at each of the rapids. A stream, with the present 
volume of the Mississippi, and its comparatively low gradient 
of about six inches per mile, can scarcely do more than 
remove the material brought in by its tributaries, to say noth- 
ing of removing the great amount of material deposited at the 
Wisconsin stage of glaciation. There appears, however, to 
have been a long period succeeding this sand deposition in 
which the volume of the Mississippi was much greater than at 
present, and this matter will next receive our attention. 
EROSION ACCOMPLISHED BY THE LAKE AGASSIZ OUTLET. 
Following this period of sand deposition the Mississippi 
valley afforded a line for the discharge of a large area now 
tributary to Hudson’s bay, an area which was occupied by the 
glacial lake, Agassiz. The area of this glacial lake, and of the 
country tributary to it, is estimated by Upham to have been 
from 350,000 to 500,000 square miles.* This great drainage 
area has been reduced to about 12,000 square miles f now 
tributary to the Mississippi through the Minnesota river. The 
present drainage area of the Mississippi, above the lower 
rapids, does not exceed 125,000 square miles, or about one- 
third the minimum estimate of Upham for the area of Lake 
Agassiz and its tributaries. Although this great reduction 
has been in the arid portion of the old drainage basin, it must 
greatly affect the volume of the river. The present run-off of 
that region can scarcely furnish a full index, since the ice sheet 
was also a great contributor of water to the glacial lake, t 
It can scarcely be questioned that at the height of the dis- 
charge from Lake Agassiz the volume of water was fully four 
times that of the present Mississippi. This view is sustained, 
both by the amount of erosion which took place, and by the 
low gradient reached by the stream. The sand which was 
deposited as a glacial outwash, while the ice sheet occupied 
the headwaters of the present Mississippi, was largely 
*“The Glacial Lake Agassiz,” by Warren Upham, Monograph XXV, U„ S. Geol. 
Survey, 1895, pp. 50-64. 
+ Warren’s Report Bridging Mississippi River, Chief of Engineers U. S. Army, 
1878-79, Vol. IV, p. 924. 
$ In addition to the change of drainage area involved in the Glacial Lake Agassiz, 
it is necessary to take into consideration the influx of water from the glacial lake 
which occupied the western end of the Lake Superior basin, and also a small glacial 
lake at the head of Green Bay in Wisconsin. 
