A NATIONAL PLAN FOR AMERICAN FORESTRY 397 



the mountains. On the average, the precipitation in the forested 

 regions probably reaches 30 inches. Most of this comes in the form 

 of winter snows. Wide variations in the precipitation occur from 

 year to year and from season to season, variations of 50 percent of the 

 normal being not uncommon. 



The thaws are mainly responsible for local floods. Very appreciable 

 differences in run-off occur from year to year, due as much to varia- 

 tions in rate of thawing as to the depth of the snow. As indicated 

 in the introductory statement to this section, the rate of snow melt is 

 affected by the condition and character of the cover, and hence the 

 spring flood flows. 



The floods in this upper basin usually have little relation to those 

 in the Mississippi, since by the time the high water reaches the Mis- 

 sissippi, the main floods of that river have already passed on to the 

 sea. They are important locally, however, primarily because of their 

 effect on sustained stream flow. Excessive spring floods affect 

 adversely the summer flow. The agriculture of a very large part of 

 the upper plains region is dependent upon stream flow from the 

 forested areas. In some years, practically all the summer flow is 

 taken for agricultural use. The forested area provides all or prac- 

 tically all the water used by such cities as Denver, Cheyenne, and 

 Rapid City. Any condition that adversely affects sustained stream 

 flow from this area, therefore, reacts to the detriment of regional 

 welfare and prosperity. 



Ordinarily the streams in the high mountain area are clear, fed by 

 thousands of forest springs and flowing through numerous lakes. 

 They become muddy or silt-laden only during the period of heavy 

 run-off. Streams rising in the more impoverished soils of the wood- 

 land areas are usually not clear, except those coming from high 

 plateaus. 



FOREST COVER 



The forest belt stretches along the mountain slopes at the western 

 edge of the basin, and takes in the mountain outposts. Although the 

 forest belt includes about 6K percent of the area of the upper basin, 

 about a third of it is nonforested lands largely range lands, such as 

 meadows, parks, and grassland plateaus. 



The higher elevations are taken by spruce forests, usually dense 

 and maintaining a heavy litter cover. On steep slopes, the forest 

 may be open, but where soil exists, minor vegetation covers the 

 ground. 



The lodgepole pine forest is usually fairly dense. Where the lodge- 

 pole gives way to ponderosa pine, the forest usually becomes more 

 open. 



Ponderosa pine is the principal species of the mountain outposts 

 and sand hills. Owing to the lower precipitation of the plains region, 

 the trees are often of low stature and the forest open but a good ground 

 cover is found where grazing, fire, and cutting have not been too 

 heavy. 



The brushy or woodland forest seldom forms a complete cover, 

 reflecting the semiarid conditions under which it has developed. 



PRESENT CONDITION OF THE FOREST 



For the most part, the forest cover of today is relatively little 

 different from that which existed prior to the advent of the white man. 



