196 OUTLINES OF FORESTRY. 



The running of the water from a higher to a 

 lower level is called drainage. 



There are two kinds of drainage : 



1. Surface drainage, where the water runs di- 

 rectly off the surface into the rivers. 



2. Underground drainage, where it first sinks 

 into the ground and afterwards discharges as 

 springs into the rivers. 



Underground drainage takes place slowly. Sur- 

 face drainage takes place rapidly. 



The direction in which rivers flow depends on 

 the direction in which the land slopes. 



The main stream, with all its tributaries and 

 branches, is called the river system. The land 

 which drains into a river system is called the river 

 basin. The size of the river depends on the ratio 

 between the quantity of the rainfall and the size 

 of the river's basin. 



"When a river basin is covered with a loose, por- 

 ous soil, such as it will have when covered with 

 almost any form of vegetation, the rain sinks 

 slowly into the earth and the river seldom over- 

 flows its banks. 



When the area is such that most of the water 

 runs directly off the surface, as will generally be 

 the case when deprived of its vegetable covering, 



