ILLINOIS RIVER DRAINAGE BASIN. 517 



courses, a feature indicating a greater maturity of drainage than is charac- 

 teristic of the Wisconsin drift; for, as already indicated, several of the large 

 tributaries of the Illinois within the limits of the Wisconsin drift have their 

 most rapid descent in the lower portion, the headwater portion being slug- 

 gish and imperfectly drained. These contrasts in drainage are only in part 

 due to the natural advantages possessed by Spoon River, and they furnish 

 an impressive line of evidence of the difference in the age of the drift sheets. 



SANGAMON RIVER. 



The Sangamon River has a larger watershed than any other tributary 

 of the Illinois. However, its drainage area, estimated at 5,670 square 

 miles, includes extensive plains in central Illinois which are inadequately 

 drained, but which may, by extensive ditching, be drained into it. 



The length of the river is about 180 miles. Its source is in the 

 Bloomington morainic system in eastern McLean County, at an altitude of 

 about 850 feet above tide, or about 430 feet above its mouth (the mouth 

 being 419 feet). In the first 10 miles it makes a descent of 120 feet, thus 

 leaving about 300 feet of fall for the remaining 170 miles of its course. 

 The fall is far from regular, there being sections, often several miles in 

 length, in which it is slight, between which are sections with more rapid 

 fall. Thus in its course through Sangamon County, a distance of 36 miles, 

 it falls only 38 feet, while in crossing Menard County, immediately below, 

 it falls 67 feet in a distance of 30 miles, and in crossing Macon County, 

 just above Sangamon, it falls 50 feet in about 30 miles. In the lower 23 

 miles, where it crosses the Illinois bottoms, its fall is only 16 feet. 



The main stream flows for about 90 miles within the limits of the 

 Wisconsin drift, leaving that drift a few miles west of the city of Decatur. 

 In this portion of its course it receives no tributaries of importance, its 

 immediate watershed being only 15 or 20 miles in width. As previously 

 indicated, it follows the west border of the Cerro Grordo moraine for fully 

 30 miles below the village of Mahomet, and that moraine constitutes the 

 east border of its watershed. Its channel is but 20 to 30 feet in average 

 depth in the first 60 miles of its course, but in the next 30 miles, in which 

 it crosses the Shelbyville morainic system and the elevated inner slope of 

 the moraine, it has trenched to a depth of 75 feet or more. Upon leaving 

 the Shelbyville system it again enters a shallow valley, scarcely 50 feet in 



