THE TOPOGRAPHY AND HYDEOGRAPHY OF ILLINOIS Xxili 



phenomena in the state. That portion of the state south of 

 the Ozarks forms part of the coastal plain which borders the 

 Atlantic and Gulf coasts. It has all the peculiarities of this 

 plain, since it is level, sand}^, and covered with residual soils. 

 It is almost entirely drained by the Cache and Big Bay rivers, 

 principally the former, whose current, owing to a reef across 

 the channel near Ullin, is very sluggish. 



THE RIVER SYSTEMS 



With these general principles in mind we come to a more 

 detailed description of the drainage basins of the principal 

 streams. Nearly the entire surface of the state is drained by 

 two sets of streams, viz: the Rock, Illinois, Kaskaskia, and 

 Big Muddy rivers, direct tributaries of the Mississippi, whose 

 general direction is southwest; and the Saline, Little Wabash, 

 Embarras, and Vermilion, tributaries of the Wabash and through 

 it of the Ohio and Mississippi, whose general direction is south- 

 east. The drainage basins of these streams will now be described 

 in order. 



Rock River System 



The Rock River system drains a part of southern Wisconsin 

 and most of the northwestern corner of Illinois. Its basin 

 covers an area of almost 10,820 square miles — 5,510 in Wisconsin 

 and 5,310 in Illinois (Leverett). This drainage basin is 40 to 

 50 miles wide in Wisconsin, but near the state-line it reaches a 

 width of about 80 miles. It narrows again in Illinois to 40 

 miles, and then to 25 miles. Its length is about 175 miles. 

 The outline thus formed is comparable to that of a pear, the 

 stem toward Rock Island. The country in this area is an 

 undulating semi-prairie region. Large expanses of unbroken 

 prairie, groves and some more extensive bodies of timber, 

 swamps, and lakes, are all to be found within its limits. Almost 

 all of the basin lying within Wisconsin is covered with drift from 

 the Wisconsin glacier, but near Janesville Rock River breaks 

 through the ''Kettle Moraine" of the Green Ba}^ lobe of this 

 glacier. South of this the basin lies in drift of lowan and 

 lUinoisan age. Although the exact boundaries of these drift 

 areas are not as yet definitely determined, the western border 

 of the lowan drift probably extends but a few miles west of 

 Rock River at any point, and for a short distance below Rock- 

 ford it follows nearl}^ the course of the river. The section of the 

 basin lying in the Wisconsin drift is characterized by extensive 



