WABASH EIVEE DRAINAGE BASIN. 531 



has an ovate form, its middle portion being- much broader than the upper 

 and lower portions. It extends on the west to the watershed of the 

 Kaskaskia, on the south to that of Saline River, and on the east to the 

 Embarras and Bon Pas watersheds. The elevation of the headwaters of 

 the main stream is about 700 feet, but the watershed falls to about 600 feet 

 in the 30 miles to Effingham, to about 500 feet in the next 30 miles to 

 Louisville, and to about 450 feet in the lower half. The elevation of the 

 mouth of the stream is 323 feet above tide. The headwaters of Skillet 

 Fork are only about 550 feet, thus giving a fall of but 100 feet in the 

 watershed in passing across the broad middle portion from northwest to 

 southeast. 



The main stream is largely independent of preglacial lines in its upper 

 40 or 50 miles, but the remainder of its course is determined by a broad 

 preglacial valley, except for a short distance just below Carmi, where it cuts 

 across a projecting spur of hills leading in from the west. This valley, like 

 other valleys in that region, has been filled in its lower course with drift 

 and alluvium to a level perhaps 100 feet above its rock bottom. It thus 

 covers low projecting points of the bluffs, and these are in some cases 

 touched by the present stream, but the spur of hills near Carmi rises 

 much above the level of the valley filling. The cause of the deflection of 

 the stream across it, like that of similar deflections on the Wabash, has not 

 been satisfactorily determined. The small tributaries of Little Wabash 

 usually enter this valley through preglacial lines, but their headwaters are 

 somewhat independent of the preglacial drainage. Skillet Fork and its 

 chief tributaries occupy preglacial valleys throughout much of their length. 



BON PAS RIVER. 



This small western tributary of the Wabash, with a drainage area of 

 about 250 square miles, is the line of discharge for a district in Richland, 

 Edwards, and Wabash counties, lying between the watersheds of Little 

 Wabash and Embarras rivers. Its course is mainly along a preglacial line 

 which has been filled in its lower course to an elevation of 60 feet or more 

 above the rock bottom. The watershed outside of the preglacial line of 

 drainage and its tributaries has only a very thin deposit of drift; hence 

 scarcely any change of drainage has resulted from the glaciation. 



