FOEEST COYEE IX PROTECTING RESERVOIRS / 
and for each combination of width, slope, and discharge there is a 
limiting coarseness of debris above which no transportation takes 
place. The rate of flow increases with the volume or the depth of 
the water in the channel. These laws of flow and of the carrying 
capacity of water at different velocities of current explain the great 
amount of erosion of soil which takes place in regions of heavy or 
concentrated rainfall, such as the mountains of California, the 
Southwestern States, and the southern Appalachian and adjacent 
Piedmont regions, regions not only of concentrated rainfall, but of 
broken surface where it is possible for surface run-off to gather 
into rapid torrents having great volume and enormous cutting and 
carrying capacity. 
The more thorough the cultivation of farming lands, the deeper 
the plowing, the more humus incorporated, and the better the granu- 
lation and the absorptive capacity of the soil, the lower will be the 
proportion of rainfall which runs off and the less will be the extent 
of the erosion. This is a matter of prime importance to the farmer 
as well as in connection with reducing the turbidity of streams, 
since, in addition to the physical destruction of land through the 
formation of gullies by erosion, the muddy effluent bears away the 
lighter and more fertile parts of the soil, resulting in its constant 
depletion. At the same time the loss of water which should soak in 
and be stored in the subsoil for future dry-season needs is often 
equally serious. Although agricultural lands contribute but a rel- 
atively inconsequential part of the total solid burden of most of 
our great rivers, except in the Southeast, on the whole the wastings 
from the agricultural lands represent the element of greatest fer- 
tility, the humus, the organic components, containing particularly 
the nitrogen or ammonia compounds. It has been computed 12 that 
an average of more than 850 pounds of soil are yearly washed from 
every acre of land on the Yadkin River above Salisbury, N. C. Of 
this more than 125 pounds are organic matter, the balance being 
mineral soil. The organic matter is humus, chiefly from farming 
soils, and where this is the case it must be replaced. 
In woodland, erosion is due chiefly to the exposure of the soil 
through the destruction of the humus and leaf litter by fire or other 
agency. A forest soil as a rule has excellent subsoil drainage due 
to root penetration, and the top soil is usually well granulated. 
Moreover, there is a prevailingly good binder of rootlets. (PL 4, 
figs. 1 and 2.) For these reasons, even when the leaf litter is an- 
nually destroyed, erosion is seldom so disastrous as it is in the case 
of a soil used for farming, or which has been farmed and abandoned. 
In open parklike woodland and brush land, such as exist in sec- 
tions where the rainfall is not sufficient to support a dense stand of 
trees, as is particularly exemplified on the lower mountains of the 
AVest and Southwest, the litter and leaves which accumulate are not 
sufficient to cover the soil and afford it adequate protection. Under 
tive to changes in fineness of debris than to changes in discharge or slope. If slope 
remains the same velocity changing with discharge, capacity for "transportation varies 
with the 3.2 power of the velocity. If discharge remains the same, in which case velocity 
changes with slope, capacity varies with the 4T0 power of the velocity. If depth remain's 
the same, the velocity changing with simultaneous changes of slope and discharge, 
capacity varies with the 3.7 power of the velocity. 
"Ashe, W. W. tekracing of farm laxds. N. C. Geol. and Econ. Survev Bui. 17: 
22. 1908. 
