The capacity of a stream to pick up and transport material is affected to a large extent by the amount of stream turbulence. The roughness of stream channel and stream velocity determines the amount of stream turbulence.
A slight increase in velocity of the stream flow results in significant increase in turbulence, which in turn accelerates the rate of erosion and also the load-carrying capacity of the stream.
Volume of Stream flow strongly affects the rate of erosion, since more water increases the velocity.
Nature of the rocks in contact with the stream directly affects its erosive work. Resistance to corrasion and corrosion of the bed rocks of the stream channel affects the rate of erosion. Where the rocks are highly resistant, little erosion takes place.
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The Slope of a stream bed or its gradient, which is expressed as the drop per unit of horizontal distance largely affects the rate of erosion, for the steeper the gradient, the greater the velocity and turbulence of the stream flow. The gradient of a stream is usually greatest at its head and in its new tributaries.
However, the greater volume of the stream flow farther downstream accelerates its velocity even more than the higher slope upstream, so that ability to pick up and carry material is equalized or even increased downstream.
Since there are many variables that affect the erosive capacity of a stream, there is variation in the erosion of the stream bed from place to place.
All streams, however, have a base level beyond which they cannot erode. At the mouth of a stream where it enters into the ocean or into a lake, the level of the larger body of water is the stream’s base level. Therefore, the ultimate base level for all stream gradation is logically the sea level.