Whenever the amounts of percipitation is in excess of the Sum of evaporation,penetration and crater collections capacity,the excess waters after formation of a layer or thin layers over the ground surface,will cause a surface flow which could have the potential of starting a river.
The water currents often flow either permanently or temporarily.Temporary flows are due to torrential showers and flooding. These currents,unlike the normal river ones, don't have any established path,instead they extend over the surface and create a flat current.
Rivers are classified into three categories based on their water resources : rivers that are supplied by,
1 - thawing of snow,
2 - rainwaters , and
3 - both.
In general every permanent and temporary current has its distinctive shape.At the source of the flow many small streams of water join each other at the foot of the elevations . The surface formed by these flowing streams resemble a cone with the base facing the peak and the vertex pointing towards the slope of the mountains.The sum of these primary cones which form even a bigger cone in the mountains is called the " basin reception point ".Small currents join into larger ones which follow in defined tracks in a valley.This valley is called water way.As these waterways encounter flat surfaces , they spread into cones with the vertex pointing towards the waterway and the base facing depression and seas.
The more or less deep,long and branching crevices that have been hollowed out in soft soil which is not compact by these rivers are called " gullies " or " ravines ".The intersection of river with sea is called the mouth.Many rivers bring with themselves sediments to the mouth and slowly form an area the shape of " Delta " whose vertex is towards the river and is called " Delta ". A river that reaches a sea is called " exodoreism " and one which flows into lagoons or lakes is known as " endodoreism ". Rivers often assume spiral shape over flat and alluvium lands which is the result of differential pressure within the two riverbanks.Sometimes these spirals recede and slowly form complete loops which are called Meander. Some other times during over flows the river may follow a straight path cutting off the loop and maintain the path up to the end of the overflow.In this situation a false river will eventually form a lagoon and then disappear.Normally,river water often flows in a small and narrow bed which is called " the minor bed ". During overflows the water level will increase to above the minor bed,cover its upper lands,and continue to flow in a wider bed called " the major bed " . Often in mountaineous regions an otherwise calm river will change in to a fast moving and roaring one called " rapides ". Waterfalls are also another phenomenon sometimes found in the path of the rivers.The reason for this phenomenon is the extension of hard bedrocks in a more or less tall stair fashion in the bed.The most important waterfalls in the world are " Niagara falls " in North America and " Victoria falls " in South Africa. Rivers often flow in an orderly fashion and in a downward slope from the highlands these to seashores and are called " consequent ". Sometimes the rivers change direction near the shore and run parallel to it . This phenomenon is called " subsequent " flow. Flow of each river goes through three stages in its life time : youth,maturity and aged. During the youth period rivers exhibit severe destruction activities and often due to great elevation differentials , higher velocities, waterfalls and craters will form.In maturity, the rivers reach an equilibrium with slower flow and minimum destruction.In old age large elevations disappear and the riverbed and surrounding area become almost flat.These periods are mostly theoretic and there are several other factors which maintain the flow in youth and maturity stages.
Adapted from the book : "Water"
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