140 
entire watershed, and will supply the river at the rate of 110,- 
000 eubic 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 been 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 
in 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. 
