SOIL EROSION IN 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 gullies on the hillsides. 
The size of the material moved by ruiming 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 ha 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. Gilbert * 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. 2 However, the maximum 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 difficult 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 finest material, or the clay particles. 
As the streamlets 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 rolled along 
1 G. K. Gilbert, U. S. Geol. Survey, Prof. Paper 86 (1914). 
2 Deacon, G. F., Inst. Civil Eng., Proc. 98, (1914). 
