108 PHYSICAL GEOLOGY 



(5) the amount and character of vegetation upon the surface, 

 and for other less important reasons. The greater the slope 

 of the surface, the larger the proportion of the rain which 

 joins the run-off, and the smaller the proportion, conse- 

 quently, which enters the ground. If the spaces between 

 the rock particles are large, as in sand or gravel, more water 

 sinks into the ground than when the surface material is dense 

 and compact, like clay. If the rocks are already nearly or 

 quite full of water, little or no more can enter. After the 

 surface material is filled with water, no more can enter until 

 that within sinks out of its way. Meanwhile, all the water 

 that falls is disposed of in some other manner. Other things 

 equal, therefore, most water sinks into the ground when the 

 downfall is gentle. This fact is in part responsible for the 

 familiar statement that gentle rains are more beneficial to 

 crops than heavy downpours. The run-off is greater from 

 bare slopes than from slopes clothed with vegetation ; in the 

 latter case, accordingly, a greater proportion of the rain 

 sinks below the surface. This fact is a fundamental considera- 

 tion in connection with the recent agitation in favor of forest 

 reserves about the sources of rivers which afford navigation 

 or water power. With such forests a larger proportion of 

 the rainfall sinks beneath the surface, later to issue gradually 

 as seepage and springs, and so maintain the volume of the 

 rivers throughout the year. Without them, the water from 

 spring rains and melting snows flows away quickly, often 

 causing destructive floods, and leaves the rivers with greatly 

 diminished volume in the dry season. 



The ground water at any given place is not dependent 

 entirely upon local rainfall, for, as noted below, water often 

 flows great distances underground. 



Depth to which ground water descends. From what 

 has preceded, it is evident that water must fill the cavities 

 in the rocks from the water table down as far as openings 

 exist. As already indicated (p. 77), this is believed to be 

 to a distance of five or six miles only, for at some such depth 



