6 BULLETIN 1430, U. S. DEPARTMENT OF AGRICULTURE 
The factors which determine the erosion of naked soils are the 
character of the soil and subsoil, the gradient of the surface, and the 
amount and character of the rainfall or other precipitation. A 
soil which is thoroughly granulated, even if situated on a moderate 
slope, is not subject to erosion. Such soils are fine gravels and coarse 
sands and soils of finer texture which are granulated on the surface 
and have thorough subsoil drainage. Soils of finer texture having 
a deficiency in binding material, such as river silts, loess, or other 
silty soils, and in the East the well-decomposed micaceous schists, 
are subject to excessive erosion. 
The character of the rainfall is also of importance. Slowly fall- 
ing rain of large total volume may, through the time allowed for 
percolation into the subsoil, be largely absorbed, leaving only a 
small amount to find its way over the surface of the soil directly into 
drainage channels. A much smaller proportion of the same amount 
of precipitation would, if concentrated, be absorbed within the same 
time. The greater the amount which is not absorbed, the greater is 
the run-off over the surface, with accompanying erosion. 
According to Kennedy, 10 the quantity of silt which can be carried 
in suspension by a stream varies according to the square root of the 
fifth power of its velocity — that is, the possible silt burden increases 
at a much higher rate than the velocity. In case the velocity of a 
stream is 2 miles an hour and it is increased during flood period to 
4 miles an hour, the quantity of silt which it can carry increases 
six times. Consequently the capacity of a stream for carrying silt 
increases enormously during flood periods. Corrasion of banks or 
channel continues to take place until the maximum silt burden for 
the velocity is reached. When there is a slacking in velocity for 
any reason, as when there are pools, or when the stream spreads out 
over a flood plain, a portion of the silt is deposited. But before a 
particle is in suspension it must be freed from associated particles 
either upon the surface of the land, upon the stream bank, or at 
the bottom of the channel. As a rule a particle must become a part 
of a bed load before it passes into suspension. So erosion as a rule 
is determined initially by the capacity of a stream for bed load. The 
weight of bodies such as sand, gravel, or stone, which can be rolled 
along the bed of a stream, varies with the velocity of the current, 11 
10 Kennedy. R. G. the prevention of silting in irrigation canals. Inst. Civ. 
Eng. [England], Minutes Proc. (1894/95), 119: 281-290, illus. 1895. Kennedy's for- 
mula was based upon studies of canals. Studies upon Texas streams made by R. G. 
Hemphill, associate irrigation engineer of the Department of Agriculture, seem to 
show approximately the same percentage of silt, 1.3 by weight, being carried on the 
Brazos River at Waco by velocities which ranged from 0.69 up to 7.50 per second, and 
from the surface nearly to the bottom. The fineness and character of the silt is a factor. 
His investigations do not seem to bear out for Texas conditions Kennedy's statement 
" that corrasion continues to take place until the maximum silt burden for the velocity 
is secured." Undoubtedly considerable additional study of this problem is necessary. 
11 Merriman, M., and Merriman, T. treatise on hydraulics. Ed. 10, rev., pp. 294, 
339. New York. 1916. According to Merriman the weight of bodies which can be rolled 
along the bed of a stream varies as the sixth power of the velocity of the current. 
Gilbert, G. K. the transportation of debris by running water. U. S. Geol. Sur- 
vey Prof. Paper 86 : 10-11. 1914. Gilbert has given the subject thorough study in the 
laboratory, but states that the results of his investigations are to be regarded only as a 
contribution to the subject. In discussing the laws which control the movement of the 
bed load he makes the following general statement : The capacity of the stream to trans- 
port debris along its bed is affected by the width of the stream, a broad channel carrying 
more than a narrow one, by the velocity of the current, the quantity varying greatly for 
small changes in the velocity along the bed. Bed velocity is affected by slope and depth, 
increasing with each factor,' and depth is affected by discharge and also by slope. The 
size of the transported particles is a factor of great importance, a greater weight of fine 
debris being carried than of coarse. But capacity of a stream to transport is less sensi- 
