140 



entire watershed, and will supply the river at the rate of 110,- 

 000 cubic feet for 16.52 days, or at half the volume for 33.04 

 days. * * * During flood stages the valley is a great lake 

 of, say, 700 square miles, into which flood waters from above 

 and from tributaries are precipitated, and from the lower end 

 of which they run out more at leisure and in reduced and 

 equalized volume." 



When we remember that even in the average year over 

 21%, or more than 8 inches, of rainfall escapes by way of 

 the river, that the greater portion of this run-off takes place at 

 times of flood, and that the overflows are greatly prolonged in 

 the lower river by the inadequacy of the channel to carry off 

 the excess of water and by the imperfect development of the 

 flood-plain consequent upon the past history of the valley, we 

 realize how important, and at the same time how unique, a fac- 

 tor is the retardation of the run-off in relation to our plankton 

 operations. 



The past decade has witnessed the completion of a vast* 

 amount of surface and under-drainage throughout large areas 

 in the watershed of the Illinois River. Extensive open ditches 

 have been dredged through localities where the slope or other 

 conditions do not favor the establishment and maintenance of 

 natural channels. These have l)een supplemented by miles 

 upon miles of tile drains, thus bringing under constant cultiva- 

 tion hundreds of square miles of territory occupied in former 

 years by pond, marsh, or meadow of the original prairie. Even 

 m the rolling prairie the thousands of little ponds and marshes 

 which formed the head waters of the various tributaries of the 

 river have been blotted from the landscape by the tile drain. 

 In addition to this the natural lines of drainage have been sup- 

 plemented in a great many cases by under-drainage, in order to 

 facilitate the run-off of the rainfall and the ground water, and 

 thus bring the soil as soon as possible into condition for culti- 

 vation. This work of drainage is to a great extent completed 

 throughout a large part of the catchment area. The principal 

 exception is the basin of the Kankakee River, but the drainage 

 of even this has already been projected. 



