462 



Irrigation and Drainage 



to hold back the water and throw down the sediment 

 before reaching this place, as shown in Fig. 149, 

 where a pit has been sunk into the porous gravel 

 below and broadened at the surface to give more area 

 for percolation through the finer material at the top. 

 There are also represented lines of underdrains leading 

 to the filter outlet, which might be needed in order to 

 bring the land quickly into the best condition. If 

 necessary, a line of such wells may be formed in a 

 surface ditch or depression, and thus increase the 

 capacity. 



THE USE OF TREES IN DRAINAGE 



In some instances where sinks without available 

 outlets are to be drained, and where the method 

 illustrated in Fig. 149 cannot be used, it is pos- 



Fig. 149. Method of draining sinks. 



sible to throw up lands of higher ground with deep, 

 open ditches between them, in the lowest portion of 

 the sink, into which the other ground may be drained, 

 and then plant water -loving trees, like the willow or 

 larch, on the sides of the ditches, where, by their 



