370 A NATIONAL PLAN FOR AMERICAN FORESTRY 



erosible. This old plain was thoroughly dissected by normal erosion 

 probably even before the advance of the ice on neighboring areas. 



Deposits of loess are a distinctive feature of the upper Mississippi 

 Basin. These are found f or ^the most part east of the Mississippi 

 River, though extensive deposits are found also in Iowa and Missouri. 

 The loess is thick about the terminals of the ice sheets in northeastern 

 Iowa and southern Illinois. Away from these borders it thins out on 

 interstream areas, although it retains its thickness along the larger 

 valleys. It occurs commonly on bluffs immediately overlooking the 

 valleys. Under a vegetative cover it is very porous and absorptive. 

 On this loess soil oaks and other upland species predominate. 



These physiographic features are largely responsible for the water- 

 shed-protective classification given the forest area of the upper 

 Mississippi River Basin. Of the total 28 million acres only slightly 

 more than 10 million acres is classed as having a considerable measure 

 of influence upon watershed conditions. The lands so classed occur 

 principally in the southern half of the basin, on loess areas and drift- 

 less areas and in the uplands of the older glaciation. In the northern 

 division, the areas of greatest influence upon watershed conditions 

 lie in the large moraines at the head of the Chippewa River. Some 

 5,700,000 acres of forest is classed as having a major watershed- 

 protective influence, and 4,430,000 acres as having a moderate 

 influence. The remaining area is considered to have relatively slight 

 influence largely because of the absorptive nature of the soil and the 

 presence of numerous lakes and swamps. The areas ascribed to each 

 class are shown in figure 7. 



The average annual precipitation totals 25 to 30 inches in Minne- 

 sota, 30 to 35 inches over the Wisconsin portion of the basin, and more 

 than 35 inches in some parts of Illinois. Most of the precipitation 

 occurs during the spring and summer months. Snowfall averages 

 between 40 and 60 inches in the north, and from 20 to 30 inches in the 

 south. Rainfall of more than 1 inch in 24 hours sometimes occurs 

 four times in a single year in the northern part. In the southern part, 

 24-hour rainfall exceeds 1 inch still more frequently, exceeds 2 inches 

 as often as once a year, and has been known to total 4 inches. 



Local floods are not uncommon. In the north, "freshets" rather 

 than major floods occur. Occasionally severe floods occur in the 

 southern streams as a result of rapid snow melt. The high percentage 

 of cleared land is a factor in these floods, because so much of the culti- 

 vated land is without a cover crop during early spring. 



HEAVILY GLACIATED REGION 



In the heavily glaciated region the character of the terrain and 

 soils makes for low run-off. In much of the region hills are either of 

 gentle slope or largely of gravel. Outwash sand plains are compara- 

 tively flat. The sands provide a large absorbing surface, and much 

 of the land characterized by heavier soils is poorly drained. The 

 stream-flow regulation effected by the many swamps and lakes is 

 supplemented by artificial reservoirs. 



Forest conditions have greatly changed in the last century. Settle- 

 ment, which began in the prairies and in the scattered oak forests of 

 southern Wisconsin, gradually spread northward and westward into 

 the more densely forested areas. Extensive tracts were cleared and 

 put into cultivation. Timber cutting followed closely upon agricul- 



