166 EARTH FEATURES AND THEIR MEANING 



of silt which is only laid down after its current has been stopped 

 on meeting the body of standing water into which it discharges. 

 If this be the ocean, the salinity of the sea Water greatly aids in a 

 quick precipitation of the finest material. This clarifying effect 

 upon the water of the dissolved salt may be strikingly illustrated 

 by taking two similar jars, the one filled with fresh and the other 

 with salt water, and stirring the same quantity of fine clay into 

 each. The clay in the salt water is deposited and the water 

 cleared long before the murkiness of the other has disappeared. 



By the laying down of the residue of its burden of sediment 

 where it meets the sea, the river builds up vast plains of silt and 

 clay which are known as deltas and which often form large local 

 extensions of the continents into the sea. Whereas in its upper 

 reaches the river with its tributary streams appears in the plan 

 like a tree and its branches, in the delta region the stream, by 

 dividing into diverging channels called distributaries (Fig. 458, 

 p. 420), completes the resemblance to the tree by adding the 

 roots. From the divergence of the distributaries upon the delta 

 plain the Greek capital letter A is suggested and has supplied the 

 name for these deposits. Of great fertility, the delta plains of 

 rivers have become the densely populated regions of the globe, 

 among which it is necessary to mention only the delta of the 

 Nile in Egypt, those of the Ganges and Brahmaputra in India, 

 and those of the Hoang and Yangtse rivers in China. 



The levee. When the snows thaw upon the mountains at 

 the headwaters of large rivers, freshets result and the delta regions 

 are flooded. At such times heavily charged with sediment, a 

 thin deposit of fertile soil is left upon the surface of the delta 

 plain, and in Egypt particularly this is depended upon for the 

 annual enrichment of the cultivated fields. Though at this time 

 the waters spread broadly over the plain, the current still continues 

 to flow largely within the normal channel, so that the slack water 

 upon either side becomes the locus for the main deposit of the 

 sediment. There is thus built up on either side of the channel a 

 ridge of silt which is known as a levee, and this bank is steadily 

 increased in height from year to year (Fig. 452). 



To prevent the danger of floods upon the inhabited plains, 

 artificial levees are usually raised upon the natural ones, and in a 

 country like Holland, such levees (dikes) involve a large expendi- 



