Deltas are accumulations of river deposits at the mouths of streams, land areas which the rivers have recovered from the body of water into which they flow. The factors which determine the formation of a delta are not altogether clear. The presence or absence of a strong tide is evidently one of these factors, for in lakes and in seas with little or no tide, almost all streams form deltas, while those rivers which empty into the open ocean almost invariably do so by means of estuaries, in which the sea encroaches on the land. In North America the rivers which discharge into the Gulf of Mexico form deltas, while the Atlantic streams nearly all have estuaries. In Europe the delta-forming rivers empty into the Mediterranean, the Baltic, and the Black and Caspian seas. Nevertheless, the tide is evidently not the sole factor in determining the presence of a delta. The Ganges and Brahmapootra have formed a vast delta in spite of the powerful tide of the Bay of Bengal; the Thames and the Rhine discharge 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; while, on the other hand, slow and moderate subsidence is favorable to delta formation.
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 excessively 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 is 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, when the unconfined waters of the river would form a lake 600 miles long, 60 miles wide, and with an average depth of 12 1/2 feet.
Fig. 99. - Delta of Rondout Creek in the Hudson River; Rondout, N.Y.
(Photograph by van Ingen).
Fig. 100. - Settling of clay in salt and fresh water, after 24 hours; the jar on the left contains salt water and is clear, while the fresh water is still turbid.
The nature of the materials of which deltas consist varies according 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 carry only 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.
The material composing a delta is stratified in characteristic ways, which vary according to circumstances. Three different kinds of beds may be distinguished: (1) the bottom-set beds, which consist of fine material, spread out in regular, nearly horizontal layers over the sea-bottom; (2) the foreset beds, which are made up of slightly coarser sediment in layers which have a decided seaward inclination, the steepness of which depends upon the depths of water in which the debris is thrown down; (3) the topset beds, horizontal layers, which are deposited by the river upon the advancing foreset strata, as the latter shoal the water, and are usually, for the most part, of subaerial origin. As a rule, the topset and bottom-set beds cover the wider area, but the foreset make up the greater volume of the delta.
When a rapid subsidence is going on, the whole delta may be submarine, and the same result may be effected by powerful wave and tidal action, as in the case of the Amazon, which has a submerged delta extending about 125 miles to sea and covered with less than 10 fathoms of water. When the rate of sinking is less than that of accumulation and the power of waves and tides is relatively small, most of the area of the delta, neglecting the bottom-set beds, belongs to the region of continental sedimentation.
The rate of delta growth depends upon the quantity of sediment supplied by the river, the depth of the sea or lake into which it discharges, the power of the waves, tides, and currents which distribute the sediment, and the stationary or subsiding character of the sea-bottom. The Mississippi delta is advancing into the Gulf at the rate of a mile in 16 years, and that of the Rhone has been built out more than 14 miles into the Mediterranean since the beginning of our era. The coast of the upper Adriatic is fringed with delta deposits which have widened from 2 to 20 miles since Roman times. The combined delta of the Ganges and Brahmapootra measures about 50,000 square miles and is still gaining, despite the powerful tides and waves of the Bay of Bengal. On the other hand, there seems to be a limit to delta growth in the sea; the Nile delta has advanced very little in the last 2000 years, for a strong current sweeps along the sea-front and carries away the sediment. Debris from the Indus extends out 800 miles from the mouth of the river, covering an area of more than 700,000 square miles.