TREES 



371 



a check on the (low of the water, and it tends to hold it ab- 

 sorbed a long time. In this wav, es])ecially in hilly and 

 mountainous countries, the forests check the rapid off-llow 

 of the water, thus preventing disastrous floods in the rivers, 

 and also making the streams of more constant volume. The 

 water seaps out slowly under the leaf mould, or is absorbed 

 by the earth and given up slowly in springs, long after the 



Fig. 150. A Denuiied Hill Shnwinf!; Effects of Erosion. 



rains have ceased. Hence the intermittent rainfall will 

 appear as a steady supply of the rivers, which therefore have 

 a more even stage of water, a very important point in the navi- 

 gation of many of our streams and in irrigation. This effect of 

 forests is perhaps the most important. In recognition of this, 

 the government has set aside great forest areas in the head- 

 water regions of many western streams. Try this experiment : 

 Place one end of a broad board or trough in a sink, or pail, 

 and raise the other end. Pour water on the upper end from 

 a sprinkler. Note how c^uickly it runs off the board. Now 



