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Our transcription: Erosion in its various forms is only one of the ways that rivers interact with the sediment and bedrock of the Earth's crust. Once this material is picked up and put into motion, it becomes part of the river's flow and is transported downstream by one of several processes of sediment transport. The shape, size, and composition of sediment influence how the sediment will be carried along in the stream and where it will be deposited. Larger particles stay near the bed of the stream and are transported by rolling, or bouncing, or skidding along the bottom. This is called "bed load." When material is moved as bed load in a stream, exactly how it moves is largely a function of size. Larger particles, things which are gravel, cobble, or boulder size, stay in contact with the bed virtually all of the time except in extreme discharge events where the velocities are very high. These particles move by rolling, or by being pushed, or by sliding along the bottom. This is the traction load of the stream, the part that is continuously in contact with the bed.
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