XXVlll FISHES OF ILLINOIS 



and the banks recede until, in Carroll county, the river enters the 

 broad bottom-lands of the Mississippi. 



Except for the headwaters of the eastern branch, the entire 

 basin lies in the driftless area and therefore maintains its pre- 

 glacial course. It has, however, received a marked accession of 

 drainage because of the blocking of a preglacial tributary of the 

 Pecatonica. This diversion occurs just below Melville, and for 

 about 3 miles below this point the stream is in a gorge but little 

 wider than its bed. The small preglacial Apple River is then 

 entered. Outcrops of the Niagara formation occur frequently along 

 the bluffs. 



PLUM RIVER 



Plum River rises in the northeast corner of Carroll county, and, 

 following a westerly and southerly direction through many windings 

 and abrupt turns, finally empties into the Mississippi in the center 

 of the western boundary of Carroll county. It rises at an altitude 

 of 900 feet but descends to 800 feet in the first 3 miles, to 700 feet 

 in the next two and one half miles, and to 590 feet in the remaining 

 32 miles of its course. The banks are often 150 feet high, and in 

 some places are very abrupt, while at other points a narrow valley 

 of one eighth to one fourth of a mile intervenes. 



The Mississippi Bluff Drainage 



Under this head are included all of the small streams of western 

 Illinois directly tributary to the Mississippi below those of the north- 

 western area. Those here briefly described are Edwards River, Pope 

 creek, Henderson River, Bear creek, Big creek, and Cahokia River. 

 The character of a multitude of others may be sufficiently inferred 

 from those of this list. The area drained by these western streams 

 includes two strips of land bordering the Mississippi, one above and 

 the other two below the Illinois basin. It consists of the eastern Mis- 

 sissippi bottoms, varying in width from one to ten miles, and of high 

 bluffs rising from 150 to 250 feet above the river, usually of loess, but 

 occasionally with precipitous rock exposures. Many of the streams 

 rise beyond this range of bliiffs on the western prairies. 



EDWARDS river 



Edwards River rises in southeastern Henry county, in two 

 branches, and flows westward through this and Mercer county. In 

 the western part of the latter it turns southward for a short distance 

 before emptying into the Mississippi about one and one half miles 



