THE TOPOGRAPHY AND HYDROGRA I'll Y OF ILLINOIS lxi 



few and comparatively short. The uplands were poorly drained 

 originally, and contained many marshes, sometimes very large, 

 and many shallow lakes. The soil here is deep, black, rich in 

 organic matter, slightly alkaline in reaction, porous, and rather 

 coarsely granulated. In the southern section the soil has been 

 washed and eroded for thousands of 3 T ears, leaving it as an ex- 

 tremely fine-grained, slightly acid residue, from which most of 

 the organic matter has disappeared. The streams of this 

 long-exposed southern area have developed themselves freely 

 in comparatively deep channels, through which their currents 

 have a sluggish flow, and have lengthened their branches back 

 to the uplands, which are thus effectual^ drained by natural 

 processes. The large streams, especially in their lower courses, 

 have formed extensive bottom-lands liable to overflow, and, 

 owing to the thorough natural drainage of the country, the 

 waters recede to a very low level during times of drought. 



Hydrographic conditions in the Wisconsin glaciation have 

 been greath" changed within comparatively recent years by large 

 drainage operations, carried on at public expense under the 

 operation of state law. Swamps, marshes, and lakes have 

 virtually disappeared, and their places have been taken by rich 

 and highly cultivated farms. Much less change has been made 

 in the lower Wabash Valley as a consequence of human occu- 

 pancy, but the original rather general covering of both lowland 

 and upland forest has been mainly removed, with the effect to 

 expose the surface to more rapid erosion than heretofore, and to 

 increase the extremes of flood and low water. 



WABASH RIVER 



W^abash River was given, by the earliest explorers, the 

 name of Ouabouskigou, said to mean " white water" in one of 

 the Indian tongues, and it bears this Indian name on the maps 

 of both Joliet and Marquette. This was later contracted by the 

 French to Ouabache, the spelling of which has since been simply 

 anglicized. The earlier explorers regarded the lower Ohio and 

 the Wabash as forming one stream, to which they gave the latter 

 name, while the upper Ohio bore either its present name or 

 that of "la Belle Riviere." 



The Wabash forms, for 198 miles, the boundary between 

 Indiana and Illinois, lying in this part of its course in a preglacial 

 valle}^, the former bed of a very much larger stream. This val- 

 ley, five or six miles across in its upper part, is filled with drift 



