506 OBSTRUCTIOlSr OF RIVER MOUTHS. 



water. Man's " improveinent " of the soil increases the erosion 

 from its surface ; his arrangements for confining the lateral spread 

 of the water in floods compel the rivers to transport to their 

 months the earth derived from that erosion even in their upper 

 course : and, consequently, the sediment they deposit at their out- 

 lets is not only much larger in quantity, but composed of heavier 

 materials, which sink more readily to the bottom of the sea and 

 are less easily removed by marine currents. 



The tidal movement of the ocean, deep-sea currents, and the 

 agitation of inland waters by the wind, lift up the sands strewn 

 over the bottom by diluvial streams or sent down by mountain 

 torrents, and throw them up on dry land, or deposit them in 

 sheltered bays and nooks of the coast — for the flowing is stronger 

 than the ebbing tide, the afiluent than the refluent wave. This 

 cause of injury to harbors it is not in man's power to resist by 

 any means at present available ; but, as we have seen, something 

 can be done to prevent the degradation of high grounds, and to 

 diminish the quantity of earth which is annually abstracted from 

 the mountains, from table-lands and from river-banks, to raise 

 the bottom of the sea. 



This latter cause of harbor obstruction, though an active agent, 

 is, nevertheless, in many cases, the less powerful of the two. The 

 earth suspended in the lower course of fluviatile currents is light- 

 er than sea-sand, river water lighter than sea-water, and hence, if 

 a land stream enters the sea with a considerable volume, its water 

 flows over that of the sea, and bears its shme with it until it lets 

 it fall far from shore, or, as is more frequently the case, mingles 

 with some marine current and transports its sediment to a remote 

 point of deposit. The earth borne out of the mouths of the Kile 

 is in part carried over the waves which throw up sea-sand on the 

 beach, and deposited in deep water, in part drifted by the cur- 

 rent, which sweeps east and north along the coasts of Egypt and 

 Syria, and lodged in every nook along the shore — and among 

 others, to the great detriment of the Suez Canal, in the artificial 

 harbor at its northern terminus — and in part borne along until it 

 finds a final resting-place in the northeastern angle of the Medi 

 terranean.* Thus the earth loosened by the rude Abyssinian 



* "The stream carries this mud, etc., at first farther to the east, and only 

 lets it fall where the force of the current becomes weakened. This explains 



