94 Irrigation and Drainage 



very materially improve their physical condition ; but 

 at the same time giving to these soils a large amount 

 of plant-food, for the material borne along in suspen- 

 sion in the water of rivers is usually very valuable, 

 derived, as it is, from the finest and best parts of fer- 

 tile soils. These ingredients of the flood waters of 

 the river Nile are extremely valuable to those desert 

 sands which, under the long action of strong winds, 

 have lost the major part of those fine and extremely 

 important grains which the sand storms of the deserts 

 have picked up and swept away. 



In the fourth type of irrigation, which is an extreme 

 case of the last, the aim is to flood low tracts of land 

 with silt -bearing water in large volume, and to hold it 

 there until the suspended matters have been deposited, 

 so as ultimately to build up the whole tract, raising it to 

 a level at which it may be naturally drained, or at which 

 a depth of fertile soil sufficient to meet the needs of 

 agriculture may be laid down over one which had been 

 undesirable. Low -lying lands have been built up by 

 this method until in the course of ten or a dozen years 

 the whole surface has been raised as much as 5 to 7 feet. 



A fifth type of irrigation, which has received a 

 notable expansion in recent years, has for its primary 

 object the rapid destruction of the organic matters held 

 in solution and in suspension in the sewage waters of 

 cities, in order that they shall reach river channels and 

 the ground -water of the surrounding country suffi- 

 ciently purified not to endanger the public health by 

 a pollution of drinking waters, or by developing un- 

 healthful atmospheric conditions. 



