The Action Of Moving Water On Soil

No soil is ever at rest. It is constantly receiving and constantly losing. The additions come mostly from the weathering of rocks and the decay of plants and animals. The losses are mostly due to the action of moving water. Moving water has been given the gigantic task of world levelling, and is working at it industriously and successfully. The mountains, hills and knolls are worn away; water carries the particles down the valleys and deposits them as soil. Lakes and ponds are being filled with soil washed from higher land. The flat lands about the lakes and streams are made mostly of soil worn away from the surrounding highlands. The streams carry great quantities of soil and deposit it in the shallows and bends. The coarser and heavier materials, as gravel and sand, are deposited first and the finer material, as clay, is deposited only when the current becomes sluggish. At the mouths of streams, where the current is sluggish, a "delta" is often formed by the accumulation of soil carried down by the streams. It has been estimated that the amount of soil carried to the Gulf of Mexico every year by the Mississippi River would cover a square mile of territory 268 feet deep.

At this rate, the American continent might be reduced to sea-level in four and one-half million years. This is but a small proportion, however, of the total amount of soil that these rivers carry, for most of it is left along their banks. According to reliable measurements, England is 550 square miles smaller now than at the time of the Norman Conquest, owing to the soft chalk and clay shores being crumbled away by waves. Every stream is constantly changing its course; many a valley farmer has had the river take away a large slice of his farm and give it to his neighbour down stream. Brooks states that within a generation the Connecticut River has gradually taken several hundred acres of rich meadow land from the town of Hadley and bestowed it upon the town of Hatfield. Smaller streams, even the tiniest rills, are transporting and building soil in a similar manner.

Sometimes this action of water is beneficial, but usually it is injurious. The loss of farm soil by erosion is discussed in Chapter XL Alluvial Soils. — The flat lands near streams are often flooded each year and receive a top-dressing of rich mud that keeps them extremely fertile. The Nile is a noted example, but many of our own rivers, including the Ohio and Mississippi, fertilise their meadows in the same way, much to the profit of man. The fertile plains of Egypt, once the "granary of the world," are not made of native soils, but of soil washed down from the mountains of Abyssinia, many hundreds of miles away. All the rich rice and cotton fields of southern Louisiana were built by the Mississippi River, of soil brought from the mountains three thousand miles away.

In some places this soil is three hundred feet deep. These various kinds of alluvial or water-built soils are among the most valuable for agricultural purposes. In any hilly country one can observe this kind of soil building going on at a rapid rate. Besides transporting soil from place to place, water also assists in soil building by wearing away the rock over which it passes. It would seem hardly possible that water should be capable of wearing away so rapidly the hardest of rocks, were it not that we can see the action going on all around us. Even a single drop, falling continuously year after year, will eat a deep hole in the hardest rock. When a volume of water is in motion, and especially when it is carrying along with it particles of soil, its grinding and filing effect is much more pronounced.

The stones on the bottom of the brook at home are rounder and smaller now than when we first watched the tadpoles there. The spring that slaked our thirst twenty years ago has worn a deeper channel in the rock over which it flows. Each year the apex of the Horseshoe Falls of Niagara is four feet nearer Lake Erie. The Colorado River, which has already worn a channel half a mile deep in the solid rock of the Grand Canon, is cutting deeper every year. All water, even the purest spring water, has some minerals and gases dissolved in it, and these help it to dissolve the rock. Rain water contains small quantities of carbonic acid gas and other gases, which increase its power to dissolve rocks.