Streams erode in various ways, depending on the nature of the channel materials and the tools with which the current is armed.
The flowing water drags on the bed and banks and also forces particles to hit the bed and banks.
These actions easily erode alluvial materials, such as gravel, sand, silt, and clay.
This form of erosion is called hydraulic action, and it can excavate enormous quantities in a short time when river flow is high.
As the banks are undermined, large masses of alluvium slump into the river, where the particles are quickly separated and become part of the stream’s load.
Where rock fragments carried by the swift current strike against bedrock channel walls, they knock off chips of rock. The larger, stronger fragments become rounded as they travel.
As cobbles and boulders roll over the streambed, they crush and grind the smaller grains, producing a wide assortment of grain sizes. This process of mechanical wear is called abrasion.
In bedrock that’s too strong to be eroded by simple hydraulic action, abrasion is the main method of erosion.
A striking example of abrasion is the erosion of a pothole.
Finally, chemical weathering removes rock from the stream channel. This is called corrosion.
We see corrosion in limestone, in particular, which develops cupped and fluted surfaces.