LOWER RAPIDS OF THE MISSISSIPPI IQ 



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 

 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. 1 This great drainage area has been reduced to about 

 twelve thousand square miles 2 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 pre- 

 sent 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. 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, Wisconsin. 



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 out-wash while the ice-sheet occupied the head waters 

 of the present Mississippi, was largely removed by the Lake 

 Agassiz outlet, throughout the entire distance from St. Paul to 

 Cairo. It is estimated that the average width of the channel 



1 " The Glacial Lake Agassiz," by Warren Upham, Monograph XXV, U. S. 

 Geol. Survey, 1895, pp. 50-64. 



2 Warren's Report, Bridging Mississippi River, Chief of Engineers U. S. Army, 

 1878-9, Vol. IV, p. 924. 



