Xlviii FISHES OF ILLINOIS 



stronger than that of the Illinois at the point where it empties into 

 it, that a delta has been formed at its mouth. 



SANGAMON RIVER 



The Sangamon River has the largest watershed of any of the 

 tributaries of the Illinois. Its drainage basin, covering an area of 

 5,670 square miles (Leverett), includes extensive plains which are 

 now inadequately drained, but which may by extensive tiling be 

 drained into the river. 



The Sangamon rises in eastern McLean county, flowing south- 

 east for about 10 miles into Champaign county, and thence south and 

 west until, in Sangamon county, it takes a northwestward course. 

 .In northern Menard county it unites with Salt creek and, flowing 

 westward, soon empties into the Illinois. The length of the river is 

 about 180 miles. Its source is in the Bloomington morainic system 

 at an altitude of about 850 feet. The mouth has an altitude of 429 

 feet, making a total descent of about 420 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. 



The river flows for its first 90 miles within the limits of the Wis- 

 consin drift, but leaves this a few miles west of Decatur. In these 

 90 miles it receives no tributaries of importance, its immediate 

 -watershed being only 15 to 20 miles wide. 



That part of the river valley lying outside of the Wisconsin 

 drift, although generally shallow, is much wider than the portion 

 within the limits of that drift sheet, and bears evidence of having 

 been opened prior to the Wisconsin stage of glaciation. The river 

 and its branches are bordered throughout most of their length by 

 strips of timber about half a mile wide on either side. 



The river is subject to great variation in volume, there being in 

 the annual flood-stages a rise sufficient to overflow banks 8 to 12 

 feet in height. At such times, being a swift stream, it probably 

 discharges not less than 15,000 cubic feet per second, and in ex- 

 treme floods the discharge probably exceeds 20,000 cubic feet per 

 second. Formerly the flow of the river was more or less regular. 

 This was due to the fact that the portion of the basin lying within 

 the Shelbyville moraine was filled with swamps which absorbed 

 the water as it fell and then gave it forth very gradually. Now, 

 however, a very complete system of tile drainage carries off this 



