286 Illinois State Dairymen's Association. 



movement, the volume of water, and the amount of sediment 

 carried in suspension. 



If a current of water with a given velocity is just able to 

 move an object of a certain size, then a current with double the 

 velocity will move an object of similar shape 64 times as large. 

 Swift streams have much greater power to wash t.iian slow ones. 

 In general, the transporting power of running water varies as 

 the sixth power of its velocity, or doubling the velocity increases 

 the carrying power 64 times, and trebling the velocity increases 

 the carrying power 729 times. Hence it is that gullies form so 

 rapidly on steep slopes and hillsides. Hills are frequently broad- 

 ly dome shaped wth a profile such that the velocity of the water 

 increases toward the brow of the hill, due to an increase, both of 

 slope and volume of water. This gives a skirt or zone of badly- 

 eroded soil, while the top of the hill or ridge has suffered com- 

 paratively little from washing. The gullies thus formed work 

 their way farther up the hill by what geologists call head erosion 

 until a point is reached where the concentration of the run-off is 

 not sufficient to produce a gully. In some cases the gullies are 

 formed by the receding of small waterfalls. This is especially 

 true where grass is growing in a draw and the gully starts in 

 from below. The fall gradually travels up the draw, the falling 

 water undermining the precipice and causing it to fall in. Gullies 

 formed in this way are usually deep and wide and very difficult 

 to fill. 



On uniform slopes, gullies may be started by very simple 

 means, such as tunnels of moles, wagon tracks and cow or sheep 

 paths, any one of which may be the very necessary small begin- 

 ning, and nature will do the rest. 



Effects of Washing. 



Nothing will completely ruin land more quickly than wash- 

 ing, especially gullying. A single season, or even a single rain, 

 may produce gullies that cannot be crossed with ordinary farm 

 implements. Unless these are promptly looked after, the land 

 soon becomes practically worthless. There are already a great 

 many fields in Illinois that have been abandoned from this cause. 



Sheet washing also damages soil greatly, as may be seen in 

 case of the "clay points," as they are commonly called. These are 

 places where washing has occurred and the top soil has been 

 largely removed, exposing the subsoil. They are usually unpro- 



