ILLINOIS RIVER DRAINAGE BASIN. 515 



or 30 feet in depth, and also very narrow flood plains. The main stream, 

 however, begins to deepen its channel before entering the large moraine, 

 and has a depth of 60 to 75 feet at the inner border of the moraine. 



In the middle course Mackinaw River has cut a channel across both 

 the large moraines of the Bloomington system and also the Shelbyville 

 moraine, with an average depth of nearly 100 feet.. The width of the 

 valley increases from about one-fourth mile in the inner part of the Bloom- 

 ington belt to about a half mile at the outer part and to nearly a mile in 

 its passage across the Shelbyville moraine. As previously indicated, its 

 fall is very rapid, being usually several feet to the mile. The river receives 

 but few tributaries in this middle course, and these drain only small areas. 

 The largest tributary is Walnut Creek, which enters from the north between 

 the two main ridges of the Bloomington system, and which drains probably 

 100 square miles. Another north tributary— Deer Creek— drains a portion 

 of the plain between the outer Bloomington moraine and the Shelbyville 

 moraine. Little Mackinaw Creek, a southern tributary, drains a portion of 

 this plain south of the river. 



In its lower course the Mackinaw River winds about in a shallow 

 channel across the Illinois Valley for a distance of about 20 miles and 

 makes a descent of about 75 feet. 



QUIVER CREEK. 



This eastern tributary of the Illinois has its entire course within the 

 limits of the valley of the Illinois River. It is the only stream of conse- 

 quence found in an area comprising not less than 200 square miles of 

 sandy bottom. It is immediately bordered by a belt of mucky alluvium, 

 averaging perhaps 2 miles in width, whose surface stands 20 feet or more 

 below the level of the bordering sandy bottom. This appears to have been 

 the former course of Mackinaw River, and was perhaps occupied also by a 

 portion of the Illinois River. By systematic ditching much of the bordering 

 districts have been drained into this channel. There are other bayous in 

 this sandy bottom which are connected with the Sangamon River through 

 Crane Creek. These also were probably once occupied by the Mackinaw 

 River, together with a portion of the Illinois. 



