FOREST IXFLUEKCES. 35 



ELEMENTS OF CONSERVATION OF WATER SUPPLIES. 



In discussing the elements of dissipation as to the degree 

 of their effect under forest cover as compared with the same 

 elements at work in the open field, we have seen that the 

 shade, the low temperature, the relative humidity, the 

 absence of strong air-currents, and the protective and 

 water-holding capacity of the forest floor are all factors in 

 the conservation of the water supplies. We have also 

 seen that the quantity of water lost by evaporation, the 

 greatest source of dissipation, may be more than six times 

 as great in the open as in the forest. The only other con- 

 servative effect of forests on water supplies is their effect 

 in retarding the melting of the snows. This acts as an 

 important function in the prevention of freshets by giving 

 the snow a longer time to melt, so that the snow water has 

 a better chance to sink into the ground. It is of course 

 more evident in evergreen than in deciduous forests. On 

 the grounds of the Minnesota Experiment Station, where 

 the woodland consists of a low growth of Oak, the snow is 

 often retained in the woods a week longer than in the open. 

 This often allows the snow water from the fields to almost 

 wholly run off before it has begun to flow from the woods. 

 Then again the daily flow of snow water from the woods is 

 much shorter than from the open fields during spring 

 weather when we have warm days and cold nights, for it be- 

 gins later in the morning and stops earlier in the afternoon. 

 Under the dense shade and mulch of the cedar swamps of 

 Northern Minnesota, the snow and ice often remain until 

 the beginning of summer. The Indians claim there has 

 never been a time when they could not find ice for their 

 sick in the cedar swamps of that section. This retarding 

 effect on the melting of snows in the spring and in pre- 

 venting the run-off is of far greater importance in the case 

 of streams that rise in the high mountains than in Minne- 



