216 



ELEMENTS OF GENERAL SCIENCE 



This same part of the current may also carry all finer grades 

 of soil material, but the slowest part of the current may be 

 so very slow as to carry only the finest particles. If a dam is 

 made across a very small stream, the heaviest materials are 

 dropped where the current is first slackened, and some very 

 fine material may not be dropped for a long time. If a tumbler 

 _^^___^^^^^^^^_^_^^^ of water which is laden 



with silt is allowed to 

 stand, an estimate may be 

 made of the time required 

 for all the silt to settle to 

 the bottom of the tumbler. 

 This will show how long 

 the water, if moving, 

 might have carried silt. 



If one digs up two 

 squares of soil of similar 

 size, but one bare and the 

 other of turf, and both are 

 then subjected to a stream 

 of water from a hose, it 

 will be seen that the turf 

 withstands the running 

 water much longer than 

 does the bare soil. Erosion 

 takes place less rapidly 

 when soil is covered or 



April 

 2-9 



April 

 14-17 



April 

 2-2-20 



Forested 



i Deforested 



FIG. 105. Effects of forests upon drainage 



Diagram to represent run-off from two sim>- 

 ilar areas in White Mountains, one of which 

 has been deforested. The vertical bars rep- 

 resent in inches the proportionate run-off. 

 Note that the forested area retains the larger 

 amount of water. From data byUnited States 

 Geological Survey 



when it is held by plant 

 roots. The roots and the 

 plants retard the water flow, as well as hold the soil. 



In wooded regions, where there is much undergrowth and 

 humus, the water from rain flows from the surface very slowly. 

 In an open field the same rain will produce quite a stream. 

 The stream upon the field is soon gone, and the surface be- 

 comes relatively dry. In the woods no stream appears for 



