8 BULLETIN 1430, U. S. DEPARTMENT OF AGRICULTURE 
such conditions and with a concentrated rainfall, or with the rapid 
melting of snowfall, erosion is active. Since it is not possible to 
maintain in such sections forest or brush growth of sufficient density 
to produce adequate litter, it is not possible to reduce erosion of soil 
below a certain amount, the amount which is fixed by the heaviest 
litter which can be maintained. (PL 5, figs. 1 and 2.) 
On the western plains there are extensive areas of unconsolidated 
soils protected only by a scant stand of grass, with scattered bushes 
intermixed. Notwithstanding that these lands are nearly level, ero- 
sion from them is often excessive. Where the grass cover has deteri- 
orated through excessive grazing, a regulation of range practice may 
have the double beneficial influence of checking further range deple- 
tion, and at the same time, by inducing the more thorough covering 
of the soil, of lessening erosion. There are undoubtedly extensive 
areas in Texas and Oklahoma where this condition prevails. 
Ditches along roads and cuts and fills along highways and rail- 
roads, especially in the Southeastern States where the rainfall is 
heavy and where grass does not readily set, contribute a large quan- 
tity of solid matter to the streams. By the use of Bermuda and 
carpet grasses in ditches, and these grasses and Japanese honeysuckle 
on embankments and slopes, much of the erosion from these sources 
could be prevented or at least greatly reduced. The rapidity with 
which erosion takes place in roads in the Piedmont and mountain 
sections of the Southeastern States is shown by the hundreds of miles 
of hollow roads in this section. A good example of road degradation 
through erosion is shown in Plate 6, Figure 2, and the deeply washed 
ditches on either side show the process by which it takes place. The 
brush in the ditch is the futile means employed to check it. 
Friction between the current and the bank is a factor in the cor- 
rasion of banks of streams. Corrasion continually takes place, espe- 
cially during periods of high water, whether resulting from rainfall 
or from melting snow. It is most active, however, in the streams 
flowing through the regions of the Great Plains. This results in 
shiftings of stream channels, which is particularly characteristic of 
the Missouri, the Arkansas, the Kansas, the Canadian, the Red River, 
and many of their tributaries. Corrasion is also not uncommon along 
many eastern streams. It is aggravated in instances by the clearing 
away from stream banks not only of all trees but of shrubs as well. 
Upon the decay of their roots the natural soil binder is destroyed. 
(PI. 4, fig. 2.) The small roots and rootlets of willows, birches, 
alders, cane, and similar trees and shrubs grow to the very water's 
edge. They are often exposed along the banks of the stream and 
form a loose basketry often quite as effective in bank protection as 
ripraps. 
In the aggregate the solid burden which is added to streams 
through the washings of ore, clay, etc., is not large in comparison 
with the turbidity which comes from other sources. Locally, it is 
of importance. This contamination of streams is sometimes per- 
mitted as a result of a sentiment opposed to interfering with the 
conduct of local industries. Waste of this kind which is deposited 
in streams must frequently be removed, often at public expense, from 
navigable channels or from reservoirs within which it has been 
deposited. In some States, as in California, there are restrictions 
against such pollution. 
