KASKASKIA RIVER DRAINAGE BASIN. 523 



OTTER CREEK. 



This small eastern tributary of the Illinois drains an area of about 100 

 square miles embraced between the mouth of Macoupin Creek and the 

 elevated rock ridge which forms the bluff of the Mississippi and Illinois in 

 southern Jersey County. It apparently follows in a general way a pre- 

 glacial drainage line having about the same watershed. 



KASKASKIA RIVER DRAINAGE BASIN. 



KASKASKIA RIVER. 



The Kaskaskia or Okaw is the principal river traversing southern 

 Illinois. With a length of 180 miles, it drains nearly 6,000 square miles. 

 Its source is in the Champaign morainic system immediately west of the 

 city of Champaign, at an altitude of about 730 feet above tide; it enters 

 the Mississippi near Chester, in Randolph County, at an altitude of 342 

 feet. Its descent is generally gradual, the most rapid section of its course 

 being in its passage through Moultrie County, where it makes a descent of 

 55 feet in about 18 miles, or 3 feet to the mile. In the headwater portion 

 there is a fall of only 110 feet in the first 50 miles. In places there are 

 pools several miles in length, the most conspicuous of these being found in 

 St. Clair County, where, in a distance of 20 miles, the fall is scarcely ] feet. 



The stream is subject to great variations in volume, for it drains a 

 region in which the substrata are of compact clay, which promotes a rapid 

 run off, and furnishes but little water in seasons of drought. A rise of 20 

 feet in its lower course is not rare, and its flood plain has been built nearly 

 to that height above the stream bed. 



The upper 80 miles of this stream lies within the limits of the Wis- 

 consin drift. The stream emerges from the Shelbyville moraine at the city 

 of Shelbyville. In this headwater portion there are no noteworthy tribu- 

 taries, and the watershed has a breadth of only 10 to 20 miles. The 

 channel is narrow and shallow from the source down nearly to the inner 

 border of the Shelbyville morainic system. There it becomes deeper, with 

 a narrow trench having an average depth of nearly 75 feet. Near its point 

 of emergence from the Shelbyville system two railway bridges extend from 

 bluff to bluff, thus avoiding the necessity for a descent into the valley, and 

 yet the bridges are only about one-fourth mile in length. 



