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river - 1 Running Water The Action of Rivers I Introduction...

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1 Running Water The Action of Rivers I- Introduction: Water resulting from precipitation on land, represented mainly by rain water, follows several paths before reaching oceans or seas, or before being recycled back to the atmosphere. Therefore, the equation: Rain water = Running water (in streams and rivers) + underground water + water absorbed by plants + water in lakes (part of which returns to the atmosphere by evaporation) is a reasonable representation of what happens to running water. Among these different venues for water, it is clear that running water, or that in rivers and streams, has the greatest effects on landscape, with its strong erosional and depositional powers. A river or a stream is defined as a form of channelized flow of water on the surface of the earth. The erosional and depositional powers of a river depend on a number of factors, the most important of which are the amount of available water, the topography, and the nature, size and amount of "sediments" or load this river can carry. II- River processes: River processes are known as " fluvial " (e.g. fluvial deposits for deposits related to running waters, ..etc.). Like winds, rivers are capable of a number of processes (1) Weathering : Which includes the breaking up of rocks into particles of different sizes before their transportation over variable distances. Weathering ( + erosion ) by the action of running water is known as corrasion ; erosion by running water that leads to the deepening of the river channel is termed vertical corrasion , whereas that which leads to the widening of the channel is known as lateral corrasion . (2) Transportation : Transportation takes place by traction, saltation, suspension or dissolution. (3) Deposition: Material transported by a river and deposited in a place different from where it formed is termed alluvium . In order to better understand these processes, and the geomorphological features that they produce, we must first examine the factors that affect stream erosion and deposition. III- Factors affecting the erosional and depositional powers of a stream: (1) Velocity : The velocity of a river or a stream affects the way it flows. In general, fluid flow is either laminar (where flow lines remain parallel) or turbulent (where such lines cross each other, Fig. 1). Higher velocities result in turbulent flow, whereas slow moving rivers are characterized by laminar flow. Clearly, the type of flow has a strong effect on the interaction between water and bedrock, and hence on the erosional and depositional powers of a stream. However, keep in mind that velocity is not the only factor affecting the type of flow. Other factors include the roughness of the stream bottom and the depth of the stream channel. (2) Discharge : The discharge of a stream is the volume of water that flows past a certain point in a specified unit of time.
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