Erosion occurs when there is a progressive reduction in a particle's size by a moving force. River erosion is accomplished in different ways: hydraulic action, abrasion, attrition and solution. Different types of erosion erode different parts of the river, such as the bed or sides of the river, as well as the particle size and type eroded.
Vertical and Lateral Erosion
River erosion occurs vertically or laterally. Vertical erosion erodes the bed and lateral erodes the river sides. Vertical erosion is caused by the pull of gravity on the water down the river, carrying erosive material from the river source through valleys, interlocking spurs and finally, to the river bed where the load is deposited. However, vertical erosion only occurs in the upper parts of rivers. The lower portions of rivers are eroded by lateral, or sideways, erosion. Lateral erosion primarily differs from vertical erosion in that it widens rather than deepens the river bed.
Hydraulic action is simply the movement of water over river bed particles. Interestingly, this vertical erosion of the bed can work itself beneath the river banks. Thus, although this is not lateral erosion, it still affects the banks by causing their collapse. The eroding effects of hydraulic action increase with the velocity of the moving water, although these effects are usually limited to the erosion of fine gravel, sand and other minute particles.
Abrasion is the most effective type of erosion, but it depends on a substantial load to work effectively. It reduces the size of particles transported by hydraulic action. The degree of erosion increases with the size of the particles and the occurrence of storms. When the particles erode to a small enough size small, they in fact do not erode the river bed but instead polish it smooth.
Attrition and Solution
Attrition occurs from the source to the river bed. Particles are made smaller as they travel; they are sharp at first but round out eventually, depending on what they make contact with, either the channel bed or other particles. Attrition is responsible for the general pattern of each river containing larger particles upstream and smaller particles downstream.
Carbonic acid is strong enough to reduce limestone to a solution. Most rocks are somewhat soluble, but limestone, or carbonate, is particularly susceptible to carbonic acid. The solution then washes down the river.