107 
east of Havana. The elevation at this place is 169 feet greater 
than at Havana and these complex drift deposits are of corre- 
spondingly greater thickness, the underlying rock floor being 
found at an elevation above the sea of 396 feet, approximately 
the level of the rock floor at Havana. 
The flood-plain of the older river lies from 30 to 50 feet 
above that of the present stream and varies in width from 5 to 20 
miles,reaching its maximum a short distance above Havana, and 
being widest upon the eastern side of the river. The bluffs rise 
from 30 to 300 feet above the present flood-plain, the minimum 
height representing only the first bluff, or that of the present 
bottoms, the bluff of the second bottoms being found further 
inland. The greater heights are reached where the first and 
second bluffs coincide, as is frequently the case upon the west- 
ern side of the river. The height of the flood-plain of the 
Illinois River ranges from four to sixteen feet, varying with 
the local conditions, the highest levels being found where 
tributary streams cross the bottom-lands. The detritus which 
they carry is deposited in large quantities in the impounded 
and quieter waters of the bottoms, and builds up the banks, 
especially below the region traversed by the tributary streams. 
This is very apparent at Havana, where the banks of Spoon 
River rise about sixteen feet above low water. The bottom- 
lands below this tributary are submerged only at maximum 
floods, while those above consist in large part of marshy tracts 
six to eight feet above low-water level. 
The total area of the bottom-lands from Utica to the 
mouth of the river, a distance of 227 miles, is estimated by 
Cooley (91, p. 61) to be 704.3 square miles. The average 
width is 3.1, of which .6 of a mile is estimated to be marsh or 
water. The middle region, extending 59.5 miles—from Cop- 
peras Creek dam (16.8 miles above Havana) to La Grange dam 
(42.7 miles below Havana),—has an average width of 4.5 miles 
and a total area of 256.9 square miles, of which 20.60 are water 
and 28 marsh. Of the 75 square miles shown in the map of the 
field of operations of the Biological Station ( Pl. IL.) 56.5 belong 
