FOEEST COVER IX PROTECTING RESERVOIRS 21 
Valley there are a few streams subject to nearly the same influences, 
such as the Pearl River of Mississippi, which may appropriately be 
included. 
Fifty-seven of the Appalachian streams have 9,241.5 miles of 
navigable water. Most of them are navigable many miles above the 
tidal limit. The power which can be developed from these rivers 
is in many instances as important as their use for navigation, espe- 
cially in those regions which are remote from coal mines. 
The most typical streams rise at high elevations, 3,000 to 5,000 
feet, in the Appalachian Mountains where they are fed by many 
swift tributaries. Leaving the mountains, they flow less rapidly 
through the hill country of the Piedmont Plateau or the rolling farm 
region of the Mississippi Valley. There is an entire absence of 
lakes and natural reservoirs. The forest at present covers about 
one-third of the area of the different watersheds. 
Except in the most rugged mountains, the soil mantle over the 
greater portion of this region is prevailing deep — in many sections 
from 20 to 50 feet. The soils may be separated into three classes : (1) 
Loams and sands which are fairly permeable and have high storage 
capacity: (2) silts which are less permeable and have a lower 
storage capacity, and which, on account of their friability, are sub- 
ject when exposed to most destructive erosion (pi. 14, fig. 1) : (3) 
compact clays deficient in granulation which have an extremely low 
absorptive power and a low storage capacity and which erode badly 
but seldomly destructively. 
The lighter soils are the most extensive in mountains and are the 
sources of the steadiest perennial springs. The heavy clays are 
extensively distributed and cover thousands of square miles on the 
Piedmont Plateau. 
The unfavorable effect of the prevailing close-textured soils upon 
stream flow is further increased by the conditions governing pre- 
cipitation. The snowfall is light, except in the high mountains, and 
the annual precipitation of 45 to 60 and even 80 inches in some 
localities frequently falls in concentrated showers. At several places 
as much as 9 inches is recorded as falling in 24 hours and at one 
place 22 inches within two days. Such torrential rains are destruc- 
tive to steep slopes which are not protected either by a well-matted 
sod or by forest. 
Below the mountains the humus, both in forest and in farming 
soil, is generally deficient, the deficiency being greatest farthest 
south. This lack of humus is partly due to the climate and soil, 
which favor the rapid oxidation of humus and partly to fires and 
grazing, which cause its depletion in the forest. The climate is 
mild, the winters are short and frequently open, and the ground is 
not covered by snow. The summers are long, the humidity is often 
high, and the rainfall is irregular and concentrated. These condi- 
tions retard the formation of humus and favor its rapid destruction. 
The proportion of cleared land on the watersheds is apparently not 
excessive, but the condition and situation of much of it tend to 
jeopard not only the value of the rivers but the permanency of the 
soil as well. L^nfavorable soil conditions have become far more 
general since 1880. The cultivation of extensive areas of hill coun- 
try below the mountains, especially of the red clays in the extreme 
