28 BULLETIN 1430, U. S. DEPARTMENT OF AGRICULTURE 
River drainage, which are a heavier rainfall but more regularly dis- 
tributed and not so concentrated and a lower evaporation factor. 
These are conditions which insure the formation of a dense sod 
wherever the humus cover is broken and there is sufficient light. 
Erosion consequently is not nearly so active in this region as in those 
portions of California where the rainfall is more irregular and 
where sod and dense herbaceous ground cover do not generally set 
on naked soil. 
COLUMBIA RIVER AND THE EXTREME NORTHWEST 
The precipitation near the northwest coast, which is the heaviest 
in the United States, reaches the maximum on the west slope of the 
Coast Range Mountains and chiefly affects small streams which drain 
directly into the ocean. Many of these are subject to high floods, 
especially during the winter and early spring, the seasons of heaviest 
precipitation. Notwithstanding the prevailingly porous and granu- 
lar soils, a soil cover is necessary to prevent excessive erosion. "When 
the timber is removed, most of these soils naturally set to sod and 
brush, but where this is not the ease the protection afforded by forest 
cover with its prevailingly excellent humus is necessary to prevent 
erosion on the steeper mountain slopes. Notwithstanding the differ- 
ence in the seasonal distribution of rainfall, the conditions closely 
resemble those of many of the tributaries of the Tennessee River 
which flow from the Smoky Mountains. 
This, however, is not the case with the Columbia River, the salient 
feature of which is its clearness and the relative evenness of its flow, 
notwithstanding flood crests of 60 feet above low water. The streams 
which most closety resemble it in the United States are those of New 
England, but the flow of the Columbia is probably more uniform 
than that of any other large stream of the United States unless 
largely fed from lake storage. This condition is due to several 
causes : the prevailingly light character of the precipitation, espe- 
cially over the Plains region where the soils are largely unconsoli- 
dated and to the covering of vegetation, largely brush and grass even 
on the plains ; the granular character of the soils in the mountains ; 
the large proportion of the run-off which comes from snow fields and 
glaciers and which takes place during midsummer, thus supple- 
menting the run-off from rainfall which largely takes place during 
winter and spring, and which is rarely so concentrated as over the 
southern portion of the Great Basin, over the southern part of the 
Great Plains, and in New Mexico and Texas. There is also some lake 
storage on the Canadian drainage. 
The heavy rainfall of the coast decreases on the eastern slopes of 
the Cascades and in the Plains areas is less than 15 inches, rising as 
the Rocky Mountains are approached to 20 inches and attaining in 
the mountains of northern Idaho and on the headwaters of the Clark 
Fork a maximum of 35 inches, and on the headwaters of the Lewis 
Fork a maximum of 30 inches. This precipitation is relatively slight, 
and it is of a character to render it available for soil absorption; 
also the prevailing surface conditions are highly favorable. The 
precipitation during the summer is extremely low compared with 
that during the winter, but is largely in the form of gentle showers, 
