Pothole

Pothole is a vertical, cylindrical erosion feature in a riverbed (most commonly).

Potholes have variable sizes in the range of several dm to a few meters in extreme cases and they are commonly more deep than wide. I spotted several potholes in Norway in a dry part of the riverbed, one of them is shown above.

How do they form? We need rapidly flowing water that is capable of carrying coarse sand, pebbles, or boulders that are whirled around in the depressions of the rocky riverbed which slowly do their grinding job. You can see many of them resting on the bottom of the pothole. You can even see small grooves on the sides of the pothole which show how the pebbles move once they are trapped inside the pothole.

Potholes aren’t rare but we need several things to happen in one place to see them. The river has to flow rapidly, the river should preferably flow on the bedrock, and perhaps it would be better to have a relatively easily erodable type of rock (limestone should do well). Finally, potholes have to be dry sometimes. Otherwise, we won’t spot them.

Pothole in a riverbed (more than half a meter across). Photo taken in Norway.