THE 



JOURNAL OF GEOLOGY 



JANUARY-FEBRUARY, 1899 



THE LOWER RAPIDS OF THE MISSISSIPPI RIVER 1 



In the early days of navigation on the Mississippi two 

 important rapids were found to interrupt the passage of vessels at 

 low-water stages ; one, about fifteen miles in length, being above 

 the city of Rock Island, 111., and the other, about eleven miles 

 in length, above the city of Keokuk, la. These became known 

 respectively as the upper and lower rapids. The latter are also 

 called the Des Moines Rapids, because of the situation above 

 the mouth of the Des Moines River. In both rapids the obstruc- 

 tions consist of rock ledges, yet the form or arrangement of the 

 ledges is not the same. The upper rapids consist of a succes- 

 sion of rock barriers called " chains," each usually but a frac- 

 tion of a mile in breadth, which pass across the river channel 

 and are separated by pools or stretches of slack water. The 

 lower rapids are more uniform, there being a nearly continuous 

 descent across them. The rate of descent, however, varies, as 

 shown below. In opening the upper rapids to navigation it was 

 necessary only to cut channels across the barriers, while in the 

 lower rapids a canal has been constructed. This consists of a 

 channel blasted out of the rock for a distance of three and a 

 half miles from the head of the rapids, below which a retaining 

 embankment is built on the river bed along the Iowa side to the 

 foot of the rapids at Keokuk. 



The precise length of the lower rapids is n.i miles, the head 



1 Read at Thirteenth Meeting of Iowa Acad. Science at Des Moines, December 28, 

 1898. Published by permission of the Director of United States Geological Survey. 

 Vol. VII, No. 1 1 



