Precipitation takes place in form of rainfall, snowfall and mist or fog. Part of the rainfall goes underground by percolation and is stored as ground water; another part goes back to the atmosphere by evaporation and evapotranspiration. Rest of the rainfall flows down the surface in form of run off.

This is the major source of running water. Besides, the melting of snow and ice gives rise to running water. Other sources are the seepage of undergound water in form of springs; overflows from the lakes. Sheet wash and run off concentrate to form rills, gullies and ravines. These converge to form streamlets, which unite to form streams. Further streams combine to form rivulet, those give rise to rivers. Thus, complex branching tree type drainage develops. Some rivers are perennial in which water flows throughout the year like the Ganges. A few are intermittent in which case; the source of water fails periodically. These are typical of semideserts. The third type is ephemeral, where streams flow only in response to precipitation. Most of the rivers of South India including Orissa belong to this category.

The streams which develop consequent to the rise of land mass are termed as consequent streams Subsequently, few tributary streams flowing almost parallel to the strike of the beds join the consequent streams. They are termed as subsequent streams. Some tributary streams join the subsequent streams. Some of them flow along the dipslope or resequent slope, these are termed as resequent streams Some others flow along the slope opposite to dip slope, known as obsequent streams.

Some streams are not controlled by geologic structure. These have zig-zag orientation and are termed as insequent streams.

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Few streams existed before the imposed surface relief and maintain their course by cutting the newly rising land mass. Rivers like the Indus, the Sutlej, and the Brahmaputra existed before the rise of the Himalayas.

Such streams are called antecedent streams. Some rivers join and debouch their load to longer rivers, these are tributary rivers of the major ones; like the Jira, the Ong, the lb, the Tel which are tributaries of the river Mahanadi. Similarly, larger rivers before debouching to the sea or ocean may be bifurcated or divided into many channels. These are called the distributaries like the Kathjori, the Devi, the Kuakhai, the Birupa, the Luna etc. which originate from the river Mahanadi.

Each river creates its own valley, though a few rivers flow in pre-existing valleys. Valley development takes place by valley deepening, valley widening and valley lengthening. The development of a river system can be compared with that of human life. Commonly, the rivers originate from the mountains and the highlands. In the initial stage, gutters combine to form streamlets and streams. In this region, gradient is high and valley deepening by downward cutting is prominent.

The rate of erosion is high and the eroded materials are transported easily. Numerous tributary streams join the trunk stream and the river system is developed. Valleys lengthen their course by headward erosion. The initial stage passes to the youth stage and the river system or drainage system is re-established. Headward erosion and downward cutting continues.

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At this stage, the river valley is ‘V’-shaped in this hilly terrain rapids and water falls are formed. Pot­holes develop by whirling action and grinding. The river with higher gradient captures some rivers by headward erosion. This is known as river capture or river piracy. The river, which is captured is known as captured river and the rest of it downstream is a misfit river, The point at which capturing takes place is know as elbow of capture and the gap is known as wind gap. Some tributary valleys are at higher relief than the trunk valley found mostly in preglaciated regions.

V – Shaped valley

Gradually, lateral cutting dominates over downward cutting and valley widening takes place. By this time, the river almost enters the plain and this marks the begining of mature stage. The river valley becomes more flat. The gradient and velocity decreases, as a result of which transporting power decreases and the load are deposited.

Flat valley

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River course is deflected and it flows in a curving fashion known as river meandering. In meanders, the velocity of water is more in the down stream or outerside and is less in the inner or upstream side. Lateral cutting is more in the down stream side and during flood, the river takes a short-cut course forming at- bow or horse-shoe lakes. The stages of formation of the ox-bow lake are shown in. During maturity, no land is left out undrained and any lake or swamp existed in the youthful stage, disappear. Rapids and waterfalls almost vanish from this region.

Towards the old stage, the river flows almost at the base-level of erosion that is the sea level. Down-ward cutting is almost negligible. However, lateral cutting coutinues, though at a slower rate. The river flows almost in a flat valley. Width of the river increases. Gradient is much reduced, hence the velocity is slow. During high floods, the water over-flows the valley and forms flood plains and a part of the load is deposited on them. On both sides of the river, deposition takes place at a greater rate forming low ridges that parallel the river course.

They vary in width and are known as natural levees. Meandering continues, but the valley width is many times that of the meander width. At the last statge, the river meets the sea or lake. That is known as the mouth of the river which is commonly very wide and is termed as estuary. Rarely, the river terminates in a marsh or desert.

Still rarely, it disapeears in some limestone or chalk terraines. These are loosing streams. Some rivers form a delta (the Greek letter A) by ks own deposition which obstructs the flow and the river is branched and repeatedly with braided islands. The delta once formed grows larger and larger like that of the Sunderban delta or the Mahanadi delta. The line drawing of Nile delta and landsat view of the Mahanadi delta are shown in respectively.