VARIATION IN LEVEL OF GROUND WATER. 427 



INFLUENCE OF MAN. 



Man influences the level of ground water in various ways. Probably 

 his most important effect upon this level is by the denudation of the forests, 

 by irrigation, and by cultivation. It is well known that commonly the 

 removal of the forests increases the run-off. It has been explained (p. 415) 

 that bad cultivation may increase the run-off and that good cultivation 

 diminishes it. But, also, cultivation increases greatly the amount of evap- 

 oration. All changes in the quantity of run-off, in the redistribution of the 

 run-off by irrigation, and in the amount of evaporation from the surface, 

 produce an effect upon the level of ground water. In general the effect of 

 cultivation is to lower the level of ground water. This is illustrated in 

 hundreds of localities within the area of the lake plains. In many places 

 where before cultivation began there were shallow lakes, marshes, or 

 swamps in which the ground-water level was at the surface, the ground 

 water is now 5 to 10 meters below the surface. For instance, in central 

 Wisconsin, sotxthwestern Portage County, in the vicinity of Almond and 

 Bancroft, in the gravelly and sandy outwash plains bordering the Wisconsin 

 drift, Dr. Weidman states that in consequence of cultivation the ground- 

 water level has been lowered from 6 to 12 meters, and the level of water in 

 the ponds and lakes lowered from 3 to 4 meters, so that the area of lake 

 and pond waters is not more than 25 to 50 per cent of what it was fifty 

 years ago, when settlement of this vicinity first began. In Clark County, 

 in the area of thick old drift overlying sandstone and crystalline rock, 

 since settlement began forty years ago the level of ground water has been 

 lowered from 3 to 4 meters. 



Man has further modified the level of ground water by drawing upon 

 the sea of ground water. He has sunk innumerable wells from which 

 large quantities of water have been taken. This drawing of the water to 

 the surface lowers the level of ground water. However, the effect was 

 probably comparatively slight until the developments of the last century. 

 During that period great numbers of artesian wells were bored, from 

 which very large quantities of water are taken. The inevitable result of 

 the opening of the numerous ready passages for the ground water to 

 again reach the surface must be the lowering, at least to a small extent, of 

 the level of ground water at the feeding areas. But probably the most 

 important lowering of the level of ground water is in mining and irrigation 



