90 IOWA ACADEMY OF SCIENCES. 



It scarcely needs to be stated that so great a filling has 

 greatly interrupted 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. J 



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 XX V, U. S. Geo). 

 Survey, 1895, pp. 50-61. 



+ Warren's Report Bridg-ing Mississippi River, Chief of Engineers U. S. Army, 

 1878-79, Vol. IV, p. 931. 



fin 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. 



