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 3 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 fiood-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 4.3 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 (PI. II.) 56.5 belong 



