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moraines which largely determine the position of tributary 

 streams. Extensive sand deposits are found in the basins of 

 the Kankakee and the Iroquois, and valley drift and alluvium 

 occur along the river and its principal tributaries. These 

 latter deposits along the Illinois River are very extensive, indi- 

 cating the size of the stream which formerly occupied the 

 valley and connected Lake Michigan with the Mississippi River. 



This deposit of. valley drift reaches its greatest extent in 

 a strip extending down the river from Pekin a distance of 65 

 miles. It varies in width from 10 to 20 miles, attaining its 

 maximum a short distance north of Havana. It is a sandy 

 plain, in some localities of which the wind has produced veri- 

 table traveling sand-dunes, with characteristic fauna and flora. 

 The drainage basin of Quiver Creek lies mainly in this deposit, 

 while the basins of Kickapoo and Copperas creeks, south of 

 Peoria, lie in the older drift and consist of loess-covered till. 



The basin of the Illinois thus lies in a typical prairie region 

 of the Mississippi Valley. To the north and east it is very flat, 

 but to the south and west it presents a more rolling surface. 

 The soil is a rich black loam one to four feet in thickness, 

 underlaid by boulder clay into which the streams have cut 

 their channels. The larger water courses are usually bordered 

 by strips of woodland. A very large part of the area drained 

 by the Illinois is under cultivation. During the last twenty 

 years the natural drainage has been supplemented by tile under- 

 drainage and by the dredging of open channels through large 

 stretches of flat country, the terminal water courses of a very 

 large proportion of the tributaries of the Illinois being thus 

 widened and extended, and the area of tillable land much in- 

 creased. The extension of these supplemental channels and 

 the removal of the turf by cultivation have undoubtedly a ten- 

 dency to facilitate the run-off of the rainfall and thus to in- 

 crease the suddenness and height of floods, and they also favor 

 the introduction of fragments of vegetation and particles of 

 loam and sand, thus increasing the amount of silt carried by 

 the waters of the river at times of flood. 



