144 



ill this river, and probably in the Mississippi also, has resulted 

 in a slight and gradual filling, in the Illinois Kiver amounting 

 now to twenty-five or thirty feet. 



'•'•■■ '■ Topography. 



The present lower Illinois River is an inconsiderable stream, 

 normally six hundred to one thousand feet wide, fiowing in a 

 belt of very low bottom-land extensively occupied by large 

 swamps, sloughs, and lakes. (PI. V., VI.) This bottom-land 

 is usually two to four miles in width, narrowest where it passes 

 through the Wisconsin terminal ridges near Peoria and iu the 

 narrower part of the valley towards the mouth. Its naturally 

 slow current is still further reduced by the series of govern- 

 ment dams, permitting a deposit of soft mud over nearly the en- 

 tire under-water surface. A few expansions occur, such as Pe- 

 oria Lake and Havana Lake, respectively a mile and a half-mile 

 wide, but these are shallow, and the river is gradually filling 

 them up and building an enclosed channel down through them. 

 On the other hand, the bottom-lands, although below normal 

 levels for a stream of this size, are so extensive that their gen- 

 eral filling up would require a comparatively long period of 

 time. Doubtless under present conditions of the watershed the 

 river is depositing silt vastly more rapidly than it did before 

 the original prairies were broken up and drained for cultiva- 

 tion. 



The remaining spaces between this modern fiood-plaiu and 

 the upland bluffs are occupied by the glacial flood-plain or 

 ." second bottom," thirty to fifty feetabove the river and twenty 

 to forty feet above the lower flood-plain. (PI. VII.) That 

 portion of this glacial flood-plain which occupies the great ex- 

 pansion of the central part of the lower valley contains the 

 principal sand deposits of the valley, and has been especially 

 studied by us. This lies entirely between the distant low east- 

 ern bluffs and the present flood-plain, which closely follows 

 the west bluff. It is about seventy-five miles long, extending 

 from the morainic border below Peoria to the vicinity of Mere- 

 dosia, Morgan county. It occupies the southwest part of Taze- 



