142 RIVER DEPOSITS 



The rate of delta growth is dependent upon the quantity of 

 sediment which the river brings down, the depth of water to be 

 filled, the force of tides and currents which scatter the materials, 

 and the rate of subsidence of the sea-bottom, should subsidence 

 occur. At the present rate the Mississippi is pushing its delta into 

 the Gulf one mile for every sixteen years. The delta of the Rhone 

 has advanced into the Mediterranean more than fourteen miles 

 since the beginning of our era, while that of the upper Rhone in 

 Lake Geneva has been built out one and one-half miles in the 

 same time. The Po delta in the Adriatic has added twenty miles 

 to the land since the time of Augustus, and the town of Adria, 

 then a seaport, is now that distance from the shore. In fact, the 

 whole Adriatic coast, from Trieste to Ravenna, is a delta forma- 

 tion, which has widened from two to twenty miles since Roman 

 times. The Nile delta, on the other hand, has advanced very 

 little in the same period, for a strong current sweeps along its sea 

 front and carries away the sediment. 



The combined delta of the Ganges and Brahmapootra, the 

 two largest rivers of India, is interesting as an example of a delta 

 built up against a strong tide. The area of this delta is given as 

 50,000 square miles, and it i*s still advancing, the rivers deposit- 

 ing more sediment in flood time than the sea can remove in the 

 dry season. The enormous quantity of material carried by the 

 Ganges and Brahmapootra, which far exceeds that of the Missis- 

 sippi, is probably the most important factor which determines the 

 formation of the delta, despite the scouring action of the sea. The 

 great rivers of China, the Hoang-ho and the Yang-tze-kiang, which 

 probably transport more solid matters than any other stream except 

 the Amazon, have formed an immense delta plain around the head 

 of the Yellow Sea, which owes its colour to the mud of these 

 streams. 



