Thirty-fourth Annual Convention. ^^^ 



Table 2. — Approximate Run-off for the Broken and 

 Hilly Land of Illinois. 



Run-off in 



Districts, rainfall. Rain fall. Run-off percent of 



inches, inches, total rain- 

 fall. 

 Northern — 



Minimum 21.46 5.2 • 23 



Average 33.48 14.5 43 



Maximum 47.22 30.0 63 



Central — 



Minimum 22.85 6.0 26 



Average 38.01 19.0 50 



Maximum 48.67 31.0 64 



Southern — 



Minimum 30.05 11.0 36 



Average 42.19 23.0 54 



Maximum 55.68 41.0 73 



These figures make the average run-off from the hilly land 

 in the northern part of the state 43 per cent of the rainfall, for 

 the central 47 per cent, and for the southern 55 per cent of the 

 average annual rainfall. When we remember that these are for 

 the hilly and broken land in these areas, the figures are probably 

 not too high. If the Illinois river basin, with its extensive 

 swamps and large areas of level or nearly level land, has a run-off 

 amounting to 24 per cent of the annual rainfall, it is certainly a 

 conservative estimate that hilly and broken land with perfect sur- 

 face drainage and with slowly pervious soil should have at least 

 twice as much. 



Sheet Washing and Gullying. 



The washing produced by the run-off is of two kinds, sheet- 

 washing or general surface washing, and ^^llying. In the 

 former the water is spread over a uniformly sloping surface 

 where there is little or no tendency to collect into streams. If the 

 soil and covering are uniform in character, the wearing of the 

 water will be the same for all points. Ordinarily, however, it is 

 not so simple as this. More frequently the gradient is not uni- 

 form, the general slope will contain many smaller slopes, the 

 water will collect into streams in the draws and this accumulation 

 water will give greater volume with less resistance, and conse- 

 quently the water will attain greater velocity. The washing, or 

 work, that the stream does varies with the velocity of the water, 

 and this depends upon four things : The slope, the resistance to 



