SOIL EEOsioisr iisr the south. 3 



leys are built up by the deposition of such material brought down 

 from the hills, and new valleys are formed by the gradual enlarge- 

 ment of gTillies on the hillsides. 



The size of the material moved by running water varies from the 

 finest material to the large bowlders rolled along the beds of swiftly 

 flowing streams. The amount of material that any particular 

 stream of water can carry in suspension is limited, and if that limit 

 is reached no more material can be picked up as long as the velocity 

 of the stream and the size of particles in contact with it remain the 

 same. 



The size of particles which can be carried in suspension by the 

 water depends on the surface-mass ratio of the particles and on the 

 velocity of the water. A discussion of the relation of the carrying 

 power of water to suspended material leads to consideration of the 

 special subject of suspensions or of disperse systems of matter. 



It has been repeatedly stated that the slope affects erosion to such 

 an extent that doubling the slope increases the erosive action four 

 times, and that doubling the velocity of a stream increases its trans- 

 porting power sixty-four times. GUbert^ has recently pointed out 

 that these statements are slightly erroneous. Instead of the quan- 

 tity of material moved varying with the sixth power of the velocity, 

 it varies nearly as the fifth power.^ However, the maxunum size of 

 grain or pebble that a stream is competent to move varies as the 

 sixth power. 



The factors which modify the capacity of a stream to transport 

 debris along its bed are many. Width of stream and velocity of the 

 water are factors. Both slope and depth affect velocity, and in 

 turn depth is affected by discharge and slope. Size of material 

 transported is an important factor, as much greater weight of fine than 

 of coarse material may be carried. The shape and density of the 

 material are also factors influencing the transportation. The course 

 which the stream follows also exerts an influence, the carrying effi- 

 ciency being affected by turns and curves. The viscosity of the 

 water, varying with the temperature, the friction against the banks, 

 and the nature of the dissolved or suspended material are factors. 

 The interaction of these factors on one another makes the problem 

 more diflicult to study. It is most important to note that the trans- 

 porting power is influenced most by the change in velocity. 



The first action of the fallen drops of water as they collect is to 

 carry with them some of the fin(;st material, or the clay jDarticles. 

 As the streamlcits grow, greater volume causes increased velocity 

 and a transporting power increasing to such an extent that larger 

 and larger particles are carried along in suspension or rolhid along 



« O. K. Ollbert, U. S. Oeol. Survey, Prof. Paper 86 (1914). 

 » T)<!acon, O. F., Tnst. Civil Kng., I'roc. flS, (1911). 



