406 A NATIONAL PLAN FOB AMERICAN FORESTRY 



In the lower part of the drainages about one third of the precipita- 

 tion occurs during the spring months. In Arkansas 24-hour rainfall 

 is recorded to have totaled as high as 8 l / 2 inches and exceeds 5 inches 

 not uncommonly. In the spring flood periods of 1882, 1912, 1913, 

 1922, and 1927, the rainfall in the Quachita-Ozark region ranged from 

 15 inches to 35 inches. In the plains region the precipitation during 

 the winter and spring months is only about one third of the annual 

 total. Most of the rains come in midsummer, with marked irregu- 

 larity from season to season. At Twin Buttes, Colo., 8K inches, 

 or half the total annual precipitation, .has been known to fall in 1 

 month. 



In the foothills and lower slopes of the Rocky Mountains, cloud- 

 bursts are not uncommon. This type of rainfall has been responsible 

 for much of the flood damage in eastern Colorado. The Pueblo flood 

 of June 1921 which caused damage in excess of $25,000,000 and the 

 loss of 120 lives, was caused by a rain of more than 7 inches of which 

 half fell in 1 day. 



In the Rocky Mountains proper, a great part of the total precipita- 

 tion is snow. The melting of this snow causes high water in the 

 streams but no particularly damaging floods. 



SOILS 



In this region Ashe recognized four broad types of soil in addition 

 to the alluvial soils. He described these, and estimated the propor- 

 tion of the drainage areas on which they occurred, as follows: 



1. Silts, very fine sands, and fine sandy loam, which are deficient 

 in cohesion and are eroded rapidly by heavy rams. About 60 percent. 



2. Clays and related highly cohesive soils, subject to erosion but 

 not eroded so readily or destructively as the silts. About 15 percent. 



3. Sands and similar light soils that have a high capacity for storing 

 water and are only slightly subject to erosion. Nearly 10 percent. 



4. Stony soils, occurring particularly in the mountain regions. 

 Stone fragments sometimes form a heavy mantle on the surface of 

 cleared land that greatly obstructs erosion. About 15 percent. 



Forests that occur on sandy soils are in general classed as having a 

 comparatively slight or at most a moderate influence upon watershed 

 conditions, because these soils are highly absorptive even in the 

 absence of a vegetative cover. Forests that grow on clay soils, as in 

 the rolling hill lands of northern Louisiana, are for the most part 

 classed as having a major influence because such soils are eroded so 

 readily in the absence of cover. 



FOREST COVER 



The forest area in these olrainages totals about 52 million acres, 

 forming about one third of the total area. It has three parts. The 

 Rocky Mountain area is relatively small, a narrow strip extending 

 about 300 miles along the eastward front of the mountains. Most 

 of this is in the Arkansas River drainage. The eastern forest area 

 lies largely east of the ninety-seventh meridian. The central area, 

 relatively small in extent, lies largely in the Breaks. 



In the Rocky Mountain section dense forests of pine, fir, and spruce 

 exist. On the poorer sites the forest is open. The foothills and mesas 



