THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xlvii 



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

 channel, across the Illinois valley for a distance of nearly 20 miles, 

 making a descent of 75 feet. 



This stream is one of the most variable in the state in the quan- 

 tity of water it carries, since it is subject to great floods in wet sea- 

 sons and becomes nearly dry in seasons of drought. This variable- 

 ness is due to several causes. The principal ones are its rapid fall, 

 its compact drift-beds, and the absence of headwater marshes. 



SPOON RIVER 



Spoon River rises in southern Bureau county. It flows south- 

 west for almost 100 miles, nearly paralleling the Illinois River. It 

 then turns abruptly southeastward and in 25 miles joins the Ilhnois 

 opposite Havana, about 40 miles below the mouth of the Mackinaw. 

 It drains about 1,905 square miles. All of this area except a little, 

 in the headw^ater portion, lies outside the limits of the Wisconsin 

 drift, occupying a region covered by the Illinoisan drift upon which 

 there is a capping of loess. The headwaters lie on the western slope 

 of the Bloomington and Shelbyville morainic systems. The course 

 of the main stream, and also of several of its tributaries, appears to 

 have been determined largely by preglacial drainage lines, but they 

 are not entirely coincident with these lines. 



Its valley is cut mainly in drift, but exposes rock at many points 

 along the base of the bluffs. The valley is very narrow except for a 

 few miles before it reaches the IlHnois River bluffs, where it widens 

 out to 2 to 3 miles. In the first mile of its course it makes a descent 

 of 70 feet. The fall gradually decreases until, in the last 80 miles, it 

 descends only 2 or 3 feet per mile. 



The river receives several tributaries from both the east and the 

 west, each of which has a length of 15 to 20 miles or more. These 

 tributaries are widely branching, and the entire watershed displays 

 a perfection of drainage such as does not occur within the limits of 

 the Wisconsin drift. Originally the entire basin was about half 

 timber and half prairie. The prairies are all small, covering only 

 a few square miles each, and separated by the strips of timber which 

 line the many streams. 



Spoon River is subject to great variations in its water stages on 

 account of its rapid run-off, due to the rapid descent of the river-bed 

 and the generally well-drained surface of the basin. In seasons of 

 drought, springs along the valley afford a considerable supply of 

 water, but the low-water discharge is less than 200 cubic feet per 

 second (Leverett). The current of the Spoon River is so much 



