The Work of Rivers
Rivers perform a tremendous amount of work by erosion, the wearing away of the land's surface. All river water carries dissolved minerals and tiny particles of silt and clay. When the current of the stream is fast enough, it carries sand, gravel, and even boulders by suspension and by rolling them along the river bed. Thus the greater the speed and amount of water, the heavier the load the river can carry. Part of the material carried is washed into the stream; part is removed by the river itself from its banks and bed. In large rivers, such as the Mississippi, the amount of eroded material carried to the sea amounts to hundreds of millions of tons annually.
Rivers vary greatly in the amount of water carried. In regions of ample rainfall, they flow throughout the year. They receive water directly during rains; they have many tributaries; they receive ground water, which has seeped into the land during rains; and they are constantly fed by lakes, swamps, marshes, and other wet areas. The greatest volumes of water are carried by the Amazon River of South America and the Congo River of Africa. Both drain vast areas of rainy tropical land. In dry regions, rivers usually flow only after rains, which often come in downpours causing flash floods.
Rivers are of value in different ways throughout the world. In Southeast Asia, the fertile soils of the valleys of the Mekong, Salween, Irrawaddy, and Ganges support regions having some of the world's highest population densities. In Europe, the Rhine, Elbe, and Seine, and their many connecting canals, provide a fine system of river transportation. In North America, rivers are used extensively for transportation, water power, and irrigation. The major rivers of Africa, northern Asia, and South America are generally used less than rivers elsewhere because the lands through which they flow are sparsely populated.