At the time when the earth was cooling and the mountains and seas were being formed, there used to be heavy rains. The rainwater flowed down the slopes of the hills and mountains and finally found their way to the open seas. The continuous flowing of the rain down these slopes made a sort of rut or track and these deepened with time. These then formed streams and brooks. With the passage of time they grew in size into huge rivers.
Rivers originate from the mountains as a trickle or rill. When the rain falls or snow melts, the water trickles down the sides of the mountains as a rill. Other rills join it. It becomes larger and streams that join the main river are known as tributaries. As a river is joined by more tributaries it becomes bigger and bigger. The area of land, which supplies a river with water, is called its drainage basin.
Some rivers are formed by the movement of glaciers. The uneven areas coming in the way of the glacier become plain and form a river. Some rivers originate from springs or lakes, some of the rain water sinks into the soft ground and seeps under the rocks. Finally it reaches a layer of rock which cannot hold any more water.
The water runs out at the surface to form a spring. A river then flows out from the spring. There are also underground rivers. These are sound in limestone areas. The rainwater wears away the soft limestone, making caves and hollows. A river sometimes flows down holes in the rock and continues flowing through the underground caves. At some point ir might also come back out into the open.
The place where the river originates is known as the source while the part, which joins the sea, is called the mouth of a river. A delta is the tract of land at the mouth, composed of alluvial soil deposited as the water slows on entering the sea. Some examples are the rivers Ganga, Mississippi, and the Nile.
In the upper courses, where the slopes are fairly steep, the river flows swiftly. It wears way the land, deepens the valley, carries soil along with it and rolls pebbles along the riverbed. When, however, the slope becomes less steep and the current of the water slower, the river cannot carry its entire load and the some of the sediment falls on its bed.
In the lower course, where the land becomes flatter and the current still slower, the river drops more and more sediment upon its bed. Now only the finer sediment is carried along. During floods the river spreads over the low-lying areas near its banks. As the waters gradually disappear they leave behind a layer of fine mud, called alluvial soil, which in time forms fertile soil.
Many rivers have different flows during summer and winter. The upper parts of the rivers are sometimes frozen during the cold winter months. This cuts off the water supply to the river. The melting of the ice as spring arrives brings water to the river. The monsoon rains bring the maximum water. During this time, sometimes the river overflows and floods the surrounding areas. In arid lands the rivers last only from a short period after the heavy rains. The extreme heat evaporates the water as the river meanders across the desert.
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