IOWA ACADEMY OP SCIENCES. 
75 
ing 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.* 
The precise length of the lower rapids is 11.1 miles, the 
head being at Montrose island and the foot a short distance 
above the river bridge at Keokuk. The total descent is 22.17 
feet, or very nearly two feet per mile. The rate of descent is 
greatest in the lower part, there being a fall of about four and 
one-half feet in the lower mile and nearly eight feet in the 
lower two miles, f Above this part the fall, though not uni- 
form, is less definitely broken into rapids and pools than in 
the upper rapids. Indeed, there appears to be a rock floor 
forming the river bed throughout the entire length of the 
lower rapids. 
Immediately above the head of the lower rapids a deep, pre- 
glacial channel appears, whose floor, as shown by several bor- 
ings, is 125 to 135 feet below the low water level of the river. 
This is filled mainly with blue bowlder clay up to about the 
level of the river bed. Sand, however, in places, extends to a 
depth of nearly sixty feet below the surface of the river at low 
water, as shown by the bridge soundings at Ft. Madison and 
Burlington. A pool extends from the head of the rapids up to 
the vicinity of Ft. Madison — nine miles. The depth of the 
pool in places exceeds twenty feet at low water stage, thus 
extending to about that distance below the level of the rock 
surface in the river bed at the head of the rapids. 
Below the rapids the river for four miles is in a narrow 
valley in which the depth of the d.rift filling is not known. It 
there enters a broad, preglacial valley, which has been found 
to constitute the continuation of that occupied by the river 
above the rapids, and which no doubt was excavated to a cor- 
responding depth, though as yet no borings have been made 
♦This consists of a channel blasted out of the rock for a distance of three and one- 
quarter 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. 
■iProm Oreenleaf’s report on “ Water Power of the Mississippi and Tributaries,” 
tenth census of United States, 1880, Vol. XVII, p. 60, the following data are obtained. 
“ In the first 4.800 feet from the lower lock there is a rise of 4 21 feet, then 2 22 feet in 
the next 3,600 feet, and 1 67 feet in the succeeding 3,600 feet to the middle lock, making 
the fall in ordinary low water, from a point opposite the middle lock to the foot of the 
rapids, 8.1 feet.” 
