254 IRRIGATION. 



water ; a soil that is destitute of fertility in its present 

 condition, either naturally, or as the result of damage 

 by washing or flooding, and that is so situated that it 

 can be covered with water from a muddy stream, from 

 which the load of suspended matter may be deposited 

 during a period of rest. After this has been done, the 

 clear water is withdrawn slowly, so that the newly de- 

 posited soil is not disturbed, and a new supply is let on. 



The lands that may be thus improved, are obviously 

 only those in river bottoms, or in bends of streams, where 

 damage has been inflicted by the washing of freshets, and 

 upon which water from the stream may be flowed either 

 by damming, or by the high water of floods, and upon 

 which the water may be retained by a series of banks 

 until it has served its purpose, when it may be withdrawn 

 through flood-gates or spouts in the banks. 



After the surface has been brought to a level, or to a 

 smooth, regular and not excessive slope, in one direction, 

 the arrangements for retaining the water should be made. 

 A succession of banks, as described in Chapter XII, pp. 



Fig. 136. WASTE GATE. 



126-127, will be needed. The higher the banks, and the 

 deeper the sheet of water that can be retained, the better; 

 for the more water that can be impounded, the greater 

 the burden of soil that will be deposited. The discharge 

 of water must be carefully regulated, lest the deposit be 

 stirred up by the current, and carried off by the retreat- 

 ing waters. To obviate this danger, the gates should 

 open at the top, and not at the bottom. The best ar- 



