THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS liii 



south. It is SO masked by drift that it presents the appearance of a 

 broad shallow basin rather than a river valley. It continues nearly 

 to the mouth of the ri\'er, where the width contracts abruptly to 

 about a mile upon entering the subcarboniferous limestone which 

 there borders the Mississippi Valley. The bottom-lands are subject 

 to annual overflow, and are still covered with a heavy growth of 

 timber. 



The stream is subject to great variations in volume as the com- 

 pact clay subsoil promotes a rapid run-off and furnishes but little 

 water in seasons of drought ; consequently, in summer and fall, the 

 river dwindles to a very small size. At times it may be crossed dry- 

 shod at Vandalia, where it is 60 to 70 feet wide. A rise of 20 feet in 

 its lower course is not rare in flood time, and its flood-plain has been 

 built nearly to that height above the stream-bed. 



The two principal tributaries of the Kaskaskia are from the west 

 — Shoal creek and Silver creek. 



SHOAL CREEK 



Shoal creek drains an area of about 1,000 square miles, or one 

 sixth of the entire basin of the Kaskaskia River (Leverett). This 

 area includes most of Montgomery and Bond counties and western 

 Clinton county. Shoal creek is made up of three branches known 

 as West, Middle, and East Shoal creeks. West and Middle creeks 

 unite to form the West fork, by the union of which with East creek, 

 twentv miles below, the main stream is formed. From the rise of 

 its branches to its mouth in the Kaskaskia this stream has a total 

 length of 65 or 70 miles. The watershed has a distinct southward 

 slope, the altitude at the headwaters being 700 to 750 feet, and at 

 the mouth only 400 feet. 



The three branches have each formed a channel 50 to 75 feet or 

 more in depth and nearly one fourth of a mile in average width in 

 their passage through southern Montgomery county, and a similar 

 depth is maintained as far down as the junction of the East and West 

 forks near Greenville. Below this point the valley is more shallow, 

 and the stream soon enters the Kaskaskia basin, where its bed is 

 but little lower than the basin plain. 



East Shoal creek is bordered closely on the east throughout its 

 entire length by a series of drift knolls and ridges (broken Illinoisan 

 moraines). Shoal creek passes through a break in this system of 

 ridges just below the junction of the East and West forks, beyond 

 which its course is largely independent of drift ridges. Middle 

 Shoal creek winds about among prominent drift knolls near Hills- 



