129 



Kankakee in which natural drainage channels have not as yet 

 been opened, a fact which has a tendency to further moderate 

 the run-off in the northern basin. 



The newer drift which covers the northern basin presents 

 a great variation in its structure and consequent effect upon 

 drainage. In general it is less compact than the older drift and 

 offers greater opportunity for the storage of ground water, 

 especially wherever extensive deposits of sand and gravel 

 occur. Storage in such deposits tends to equalize the run-off 

 throughout the year. 



The basins of the several tributaries present marked 

 peculiarities which influence their contributions to the flood 

 waters of the main stream. The basin of the Des Plaines 

 River is largely underlaid ])y impermeal^le rock upon which 

 the drift is twenty to one hundred feet in thickness. The 

 lower end of the watershed alone contains deposits capable of 

 affording considerable ground storage, while the upper end 

 abounds in lakes, bogs, and swamps, which also have a tendency 

 to retard and equalize the flow of the run-off. In spite of these 

 equalizing factors the floods of the Des Plaines assume large 

 proportions and, owing to the extent in latitude of the basin, 

 they are often prolonged. A considerable portion of the 

 flood water, under conditions prior to the construction of the 

 drainage canal, escaped over the Ogden dam and through the 

 Ogden-Wentworth ditch and the Chicago River to Lake Michi- 

 gan, following what seems to have been a former channel of 

 the Des Plaines. Cooley ('89) estimates that the discharge of 

 the normal extreme flood at the junction of the river with the 

 Kankakee is 12,000 cubic feet per second, and that this would 

 be increased to 20,000 if all the water from the basin sought 

 this outlet. This latter estimate is equivalent to the bank-full 

 capacity of the Illinois River at Copperas Creek dam, seventeen 

 miles above Havana. High-water level at the junction is 15.7 

 feet above low water. The flow at low water is insigniflcant, 

 amounting in 1887 to less than 16f cubic feet per second for a 

 period of five months. The variations in the Des Plaines thus 



