262 Irrigation and Drainage 



to settle, when the clear water returned to the stream 

 with the fall of the tide. 



So large was the amount of sediment carried in 

 the water, and so rapid was the silting -up, that fields 

 of 10 to 15 acres are said to have been raised from 

 one to three feet during a single season, thus convert- 

 ing worthless peat bogs in so brief a time into fields 

 of the richest soil. One season spent in warping, 

 one for the ground to settle and become compacted, 

 and a third to get it into grass, is the usual time 

 required for reclamation, and after this such fields 

 produce enormous crops of almost any kind suited to 

 the climate. In other regions, where less sediment 

 is carried in the water, or where greater depths of 

 silt must be laid down in order to secure the desired 

 level of the surface, longer time is required for the 

 work, but in Italy fields have been raised as much 

 as 6 to 7 feet in 10 years. 



In other portions - of the world, notably in the 

 Nile valley, a modification of this system of silting 

 for the yearly enrichment of the soil is practiced. 

 To this end the ancient irrigators, both in upper and 

 lower Egypt, had laid out the accessible lands for 

 basin irrigation, by which the turbid and fertile waters 

 of the Nile, at its flood season, could be led upon the 

 settling areas and held until the rich sediments were 

 laid down, thus converting otherwise comparatively 

 worthless sandy soils into the richest and most de- 

 sirable of fields, and so maintained for thousands of 

 years by periodic inundations. 



Then, again, in France, as in the Moselle valley, 



