330 A NATIONAL PLAN FOR AMERICAN FORESTRY 



sive silting of reservoirs, or of gravel beds into which run-off is sunk 

 for underground storage and later pumping, as in southern California. 



Forests considered as of moderate influence include those occurring 

 on moderate slopes, on areas of moderate or scant rainfalls, on soils 

 which do not erode readily, and on areas where the forest cover quickly 

 reclothes the land in case fire, lumbering, or other cause depletes the 

 cover. 



Forests considered as of slight to no influence include those areas 

 where it appears that the forest exerts only a poorly defined or very 

 indirect influence upon streams, w^ater supplies, or erosion. Forest 

 areas classified as slight include those more or less level areas occurring 

 on old lava flows, or on deep sandy soils through which precipitation 

 readily percolates, and from which erosion is very slight. Likewise 

 forests occurring on poorly-drained and nearly level areas have been 

 considered as of slight influence. 



In order to indicate these broad classes on the maps accompany ing 

 the report it has been necessary to include with a larger area of another 

 classification many small areas not strictly classifiable as shown. As 

 intensive a classification as possible was made, however, and is shown 

 diagramatically on the regional maps whenever the areas were large 

 enough to be recognized. 



DRAINAGE BASINS OF THE UNITED STATES 



Forest conditions throughout the United States differ so widely, 

 their influence on watershed problems is so varied, and the importance 

 and character of the watershed problems so varied, that for purposes 

 of even somewhat detailed description the country has been divided 

 into drainage regions. In part these are purely geographical, in part 

 they represent large drainage basins. Figure 2 indicates the division 

 of the United States into these regions. 



For each of the drainage regions the influence of the forest is rep- 

 resented. The present condition of the forest, and its effectiveness 

 in exerting the influence of which it is capable are also indicated. 



NORTHEASTERN DRAINAGES 



The northeastern drainages as here covered include all those streams 

 which flow into the Atlantic Ocean north of the Potomac River, 

 excepting tributaries of the St. Lawrence River. In considerable part 

 they include the two regions discussed elsewhere as the New England 

 and the Middle Atlantic States (fig. 3). 



CLIMATE AND PHYSIOGRAPHY 



Compared with many portions of the United States where water- 

 shed and stream flow problems are also acute, the northeastern drain- 

 ages receive abundant rainfall from 30 to 50 inches annually at most 

 points, and up to 75 inches on some of the higher mountains. There 

 are no conspicuously wet or dry seasons; the average monthly rainfall 

 during the wettest months of the year is rarely more than double that 

 of the driest months. 



In spite of the normally high and equably distributed rainfall, how- 

 ever, deficiencies and irregularities in rainfall are by no means uncom- 

 mon. In 1930 for example, the southern and central portions of the 



