THE TOrOGRAPUY AND HYDROGRAPHY OF ILLINOIS ll 



This variableness 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 

 southwest for almost 100 miles, nearly paralleling the Illinois 

 River. It then turns abruptly southeastward and in 25 miles 

 joins the Illinois opposite Havana, about 40 miles below the 

 mouth of the Mackinaw. It drains about 1,820 square miles. 

 All of this area except a little in the headwater 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 Bloom- 

 ington 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 Illinois 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 consider- 

 able 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 stronger than that of the Illinois at the point 

 where it empties into it, that a delta has been formed at its 

 mouth. 



