200 REPORT OF THE FORESTRY COMMITTEE 



ground water. With the destruction of the forest the ground water is greatly 

 decreased, there is no longer the retarded surface run-off from forest soil, and 

 nearly all the snow water runs off at once as surface water from the fields and 

 cleared land. 



In cultivated fields and clearings in the north the ground is still frozen when 

 the snow melts. This, together with the rapid melting of the snow in the open, 

 causes the water soon to run off, even from gentle slopes, in great quantities, as 

 though from the roof of a building. Freezing of the ground in fields and clear- 

 ings is due chiefly to unimpeded radiation in the fall and to the blowing away 

 of the protective cover of snow in the winter. Cultivated ground freezes espe- 

 cially deep during the winter if saturated with rain water at the time of the first 

 fall frost. Surface run-off from the open fields is further increased when thaws 

 during the winter coat the ground under the snow with an icy sheet, over which 

 the snow waters run off in the spring without penetrating the ground. 



In the forest, on the other hand, the soil is warmer than in the open. It is 

 protected from radiation by trees. It is further protected by the leaf litter, a 

 poor conductor of heat, which both prevents its cooling off and protects it from 

 freezing in winter, and, in the processes of fermentation and decay, contributes 

 the heat which these evolve. The relatively even cover of snow on the ground 

 protects it still further. Under this triple protection the forest soil either does 

 not freeze at all or freezes much later in the winter and to a much less depth than 

 in open places. Moreover, it thaws out in the spring while still under its cover 

 of snow. The slow melting of snow in the forest, together with the unfrozen, 

 or only slightly frozen, condition of the ground beneath, permits a much greater 

 percolation there than in the open. This water-holding capacity of the northern 

 forest is more marked in coniferous stands, especially in spruce. 



Closely connected with the relatively high temperature of the forest soil is 

 another important fact which is often entirely overlooked. If the soil of water- 

 sheds remains soft and unfrozen, the ground water which feeds the streams con- 

 tinues to flow throughout the winter, thus keeping up the normal winter water 

 stages in the streams under the ice. If, however, the flow of underground water 

 ceases during the winter, the water accumulates in the ground, small streams 

 freeze to the bottom, and the water stage of the river falls. In spring the ground 

 water which has accumulated behind the icy dams thus formed at the bottoms 

 of slopes bordering the streams enters the rivers in large quantities. For the 

 regimen of rivers, therefore, the importance of forest cover on slopes bordering 

 springs, creeks, and small streams, which are fed by underground waters, is es- 

 pecially great. 



The forest floor, penetrated by a network of roots and covered by branches 

 and stumps, offers many obstructions to the surface run-off and so permits the 

 water to sink into the ground. Percolation is made still easier by the presence of 

 deep channels in the soil, left by the decay of large roots. 



The porosity or permeability of the soil has a great influence on the amount 

 of surface run-off. The influence of the forest, therefore, will vary with the 

 character of the soil on which it grows. On heavy clay or other impermeable 



