In the last chapter we saw what huge gifts of water are handed over, year by year, from the Land to the Ocean. But the Rivers, acting as handmaidens to carry these offerings, do not bestow water only. The water-gifts are laden with solid materials, to be used for building purposes.
A river tears earth and sand from its own banks, and wears down the stones and rocks in its bed; and to this growing collection, as it flows, it adds contributions of earth and sand, brought from higher reaches by its tributary streams. Most of those materials are kept afloat.
To make sure that they are so, we only have to examine the mouth of a river. Floating mud and sand may not be apparent in the river itself; but just where the river joins the sea, just where the outflowing stream encounters the inflowing ocean-tides, a bank or banks of mud and sand may be seen. These are a common feature of river-mouths.
Many a larger river has at its mouth an enormous network of banks, divided by streams, and the whole is described as the “delta” of that river; its shape being roughly like the Greek letter “delta,” or like an opened fan. Everybody has heard of the Delta of the Nile, the Delta of the Ganges, the Delta of the Mississippi. But hardly a river exists, no matter how small, which on joining the sea does not make its own little delta.
And the formation of these deltas comes about in a very simple manner.
So long as a river flows onward, at a fair pace, it is able to hold up its floating materials, to keep the earth and sand in its embrace from sinking to the bottom. But when it reaches the sea, its progress is checked; and no sooner does it slacken speed, than some of its buoyed-up materials begin to sink. The heavier kinds are the first to touch bottom, getting piled up so as to form a bar or bank; and farther out, as the stream grows yet more slow, lighter substances find their way downward, making another bank.
In fact, the river, with Ocean’s help, is busily building Land, forming an island or a group of islands, either of pebbles or sand or mud. In the case of a great river such islands are often very extensive.
The flow of a small river is quickly stopped by the sea, but such powerful streams as the Ganges or the Nile pour onward for many many miles, before they begin to mingle with the salt water, their speed slackening gradually. Not only do they form great deltas of islands, but they carry vast supplies of material far into the ocean.
Recently an examination was made into the waters of nineteen important rivers, to discover what quantity of material was carried by each. The result of this examination was somewhat startling.
We have seen already how many cubic miles of water a river may give over yearly to the great deep. It was found that, on an average, these rivers may be reckoned to give over also, with each cubic mile of water, more than seven hundred and sixty thousand tons of material, torn from the land. These enormous collections, together with vast quantities which the waves have dug and wrenched from beaches and cliffs, are dropped to Ocean’s floor.