Explain how the shape of a river's stream bed can affect the river's speed and its power to cause erosion.

The shape of a river's stream bed affects the speed and eroding power of a river by influencing the amount of friction between the water flowing through the river and the stream bed below. Friction is the force that opposes the motion of one surface as it moves across another surface. In a river, there is friction between the flowing water and the stream bed beneath it. Where a river is deep, less water comes into contact with the stream bed, so there is less friction. The reduced friction means there is less opposition to the water's motion, so the river flows at a greater speed and has more kinetic energy for erosion. On the other hand, where a river is shallow, more water comes into contact with the stream bed, so there is more friction. The increased friction means there is more opposition to the water's motion, so the river flows at a lesser speed and has less kinetic energy for erosion. Also, the roughness of a stream bed covered with boulders and other obstacles causes the water to flow through the river in a turbulent fashion. This could cause the river to move slower in some cases or erode more in other cases.