83 



regulators were provided between them, the water from all could be 

 run at once. If one of the embankments of an upper basin l)roke, it 

 meant devastation to everything below. The basins could not be 

 emptied until the Nile began to recede, and there was nearly as much 

 danger in having the flood continue too long as in not having a suffi ^ 

 cient supply of water. This system has survived to the present time. 

 While the basins lirst laid out were crude, they have developed after 

 many years of experience into well-regulated systems. Expensive 

 regulators have been constructed and canals have been made large 

 enough to carry water to supply the land they were intended to serve. 

 The escapes into the Nile have been perfected. The land near the 

 Nile is above the level of the adjoining farms (tig. 6). For this reason 

 it is difficult to lill the basins near the Nile eml)ankments. The grade 

 of the Nile varies from one-half to one-third of a foot per mile. 

 Owing to this slight fall the canals have to be quite large, because 

 their o-nide must be less than that of the river. Even under the most 

 favorable conditions they can not gain more than a small fraction of a 



Fig. fi.— Typical cross section of the Nile Valley. 



foot per mile over the river. When a canal reaches the edge of the 

 desert, or. in other words, covers all of the arable land except the Nile 

 berm, it follows the desert until a* new canal is taken out, when the 

 first canal siphons under the new one and covers the high land along 

 the river. The second canal proceeds in the same way and siphons 

 under the third. By this system canals can be made to serve the 

 entire area of agricultural land. 



PI. VIII shows a portion of the Nile Valley in the province of Keneh 

 where the river has a general course from east to west. The strip of 

 irrigated land, bounded t)y right lines, is in no place over 7 miles wide. 

 It will 1)6 seen that the Rannan Canal heads at the right, on the south 

 bank of the river, and that the Marashdah Canal siphons under it just 

 below the point of diversion. The latter canal is on a higher line at 

 their intersection and waters the elevated lands along the berm of the 

 Nile for 12 miles below the siphon. The Rannan Canal continues 

 westerly and soon covers all the land to the border of the desert. Just 

 27752— No. 130— (13 3 



