CAUSES INFLUENCING FLOW OF RIVER. 15 



purposes by the construction of a dam at its southern end. After- 

 wards, from a belief that the land at the bottom of the lake would 

 prove exceptionally valuable for agricultural purposes, the dam was 

 removed and the tract allowed to revert to its original condition, but 

 apparently without fulfilling the expectations which had been enter- 

 tained. At present the prevailing opinion seems to be that it would 

 be wise to reconstruct the dam in order to help the regulation of the 

 tiow of the river during the critical months of July, August, and Sep- 

 tember. As the old lake covered an area of 73 square miles, and its 

 catchment amounted to approximately 400 square miles, which is very 

 nearly one-ninth of the entire Wisconsin drainage area of the river, it 

 appears likely that the husbanding of the spring freshets at Horicon 

 Marsh by means of a dam would overcome to some extent a scarcity 

 in the summer months. 



EFFECTS OF CHANGES IN THE SOIL COVER 



The other factor which has affected the rate of the water flow, the 

 soil cover, is probably much more important than the influence of 

 artificial drainage. Under this head the forest changes hold the first 

 place. The importance of these changes lies not, as is often supposed, 

 in the influence that forests may possibly have upon rainfall, but in 

 the fact that they have a decided effect upon both the evaporation and 

 the distribution of the rain after it has fallen. In the region here 

 considered the reduction in the area of the forests has probably not 

 resulted in any appreciable decrease of the rainfall. Such a change 

 may possibty have taken place to the north and in the neighboring 

 States of Minnesota and Michigan, where large and continuous 

 forest areas have been enormously depleted within the past three 

 decades; and it is conceivable that the extensive alteration of" these 

 regions may, notwithstanding their remoteness, have had some slight 

 influence on the diminished rainfall shown in the table on page 13. 

 The relation between forests and rainfall, however, has not been 

 sufficiently investigated to draw positive conclusions for particular 

 localities. On the other hand, the effects of a forest cover on evapora- 

 tion and distribution have been successf ulty studied, and are of inter- 

 est in the present discussion because a retarded evaporation from soil 

 would naturally increase the available water supply, w T hile a regulated 

 distribution of that supply would increase its effectiveness. 



INFLUENCE OF THE FOREST ON EVAPORATION. 



Investigations carried on for a number of years in different parts 

 of Europe have proved that the humidity within a forest is greater 

 than over open ground, while the temperature is lower during the 

 summer months both within the forest itself and in its soil. These 



