=The delta of the river.=—As it approaches its mouth the river moves more and more sluggishly over the flat grades, and swings in broader meanders as it flows. Yet it still carries a quantity 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 Δ 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).