30 BULLETIN 1430, U. S. DEPARTMENT OF AGRICULTURE 
SUMMARY OF REGIONAL CONDITIONS 
In the Northeast, in the region of the Great Lakes, and on the 
Columbia River, the silt burden of streams is on the whole negli- 
gible. There are no marked periods of extremely low water, dis- 
astrous floods are infrequent, and there is a low ratio of minimum to 
maximum flowage. The forest cover exercises little influence in de- 
termining the life of reservoirs. 
The streams of the Middle Atlantic States, in spite of a wide 
margin between maximum and minimum discharge, have a much 
lower turbidity than those farther south. The drainage basins of 
many of them, however, are on sandy, shaley, or very permeable soils, 
especially at their headwaters, and for this reason erosion of soil 
proceeds at a comparatively slow rate even on denuded sites. Forests 
exercise a considerable protective function. 
The streams of the Great Plains, and likewise the western Colo- 
rado River, flow through vast areas of unconsolidated soils on which, 
because of irregular and deficient rainfall, it is not possible in many 
places to maintain vegetative cover of sufficient density to obtain 
protection against erosion. Forest cover, however, exercises a high 
influence upon the headwaters of such of these streams as lie within 
the forested portions of the Rocky Mountains. The streams which 
have their headwaters in central Texas are in the same general class 
as those of the Great Plains. The rainfall, however, is more irregu- 
lar, at times torrential, followed by long periods of scant precipita- 
tion. A consequence of this is extremely erratic flow of the rivers. 
The basins of these streams are largely covered with open parklike 
stands of trees, and there is a fairly good cover of grass, which in 
this region forms a more efficient protection than the scattered i 
trees. A few areas of real forest occur, however, chiefly on the low 
mountains, and they should be carefully protected. On the whole, 
however, the protection afforded by forest cover is inconsequential. 
It is in the Appalachian and adjacent Piedmont regions from 
Ohio and Maryland to northern Mississippi, and in the similar Ozark 
region of Missouri and Arkansas, in the mountains of middle and 
southern California, and on the slopes of the Rocky Mountains south 
of middle Colorado that forests seem to exercise their maximum 
influence in the protection of reservoirs. In these regions rainfall is 
irregular and often concentrated ; soil where denuded erodes easily, 
contributing a high solid burden to streams; snowfall is light; and 
there is little lake storage. When forests are reestablished on such 
denuded lands, and reestablishment is readily effected, erosion of 
soil is largely checked. 
Table 1 shows, for typical streams in different hyclrographic 
regions of the United States, the comparative amount of turbidity, 
relation between maximum and minimum flow, and precipitation 
on the basin of the stream with maximum rainfall for each region. 
