DELTAS I4I 



Nevertheless, the tide is evidently not the sole factor in deter- 

 mining the presence of a delta ; the Thames and the Rhine dis- 

 charge into opposite sides of the North Sea, yet, while the latter 

 has built up a delta, the former opens into a wide estuary. If 

 the sea-bottom is subsiding faster than the river deposit is built 

 up, no delta will be formed, but an estuary. The Ganges and 

 Brahmapootra have formed a vast delta in spite of the powerful 

 tide of the Bay of Bengal. 



When a stream loaded with sediment flows into the relatively 

 stationary waters of a lake or sea, its velocity is checked and the 

 greater part of its load very rapidly thrown down. Deposition 

 takes place much more rapidly in salt water than in fresh, because 

 the dissolved salts reduce the cohesion of the water, and hence 

 diminish the friction which retards the settling of silt. The exces- 

 sively fine particles of clay, which in fresh water remain suspended 

 for weeks, are thrown down in salt water in a few hours ; hence 

 the great mass of the sediment falls to the bottom in the vicinity 

 of the stream's mouth. Such rapid accumulation obstructs the 

 flow of the river and causes it to divide and seek new channels, 

 especially in time of flood, and form a network of sluggish 

 streams meandering across the low flats. The height of the delta 

 is increased by the spreading waters of the river, when in flood, and 

 the growth of vegetation assists in raising the land. Though the 

 Mississippi delta is an area of subsidence, two-thirds of its surface 

 are above water, when the river is in its ordinary stages. But for 

 the levees, however, most of it would be inundated in times of flood. 



The nature of the materials of which deltas consist varies ac- 

 cording to circumstances. When mountains are near the coast, 

 the streams flowing from them may descend into the sea with 

 sufficient velocity to build a delta of cobblestones and coarse 

 gravel. Usually, however, deltas formed in seas are composed of 

 very fine materials, because the lower course of most rivers is 

 through flat plains, and the stream can only carry very fine silt. 

 Even in such cases, there will be differences in the coarseness and 

 fineness of the material, corresponding to the seasons of high and 

 low water in the river. 



