Q:

How are potholes formed in rivers?

A:

Potholes form at the bottom of rivers as eddies gouge out the bedrock of the river bottom. These eddies form as isolated whirlpools break off from the main current of the river. If the whirlpool pattern is persistent, it can dig permanent irregularities in the rock that form the initial conditions for further erosion of the pothole.

Once the small depression is present in the river bedrock, flowing water transports sand and small rocks into it. The whirling water of the eddy carries the abrasive material around the interior surface of the depression. This action scrapes the walls and bottom of the enlarging pothole and works to enlarge the feature. As the pothole grows, larger stones are carried into it and accelerate the erosion of the feature's sides. These large stones, which are called abraders, can be carried out by the same spiraling water that carries them in, or they can settle to the bottom of the pothole where they remain indefinitely.

The pothole is usually exposed as the river above it shifts its course. The water inside then drains off or evaporates away, leaving a rubble-strewn pit that can be as much as 25 feet wide and 80 feet deep.

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