Riverbeds often exhibit strange holes formed in bedrock. These “potholes” can resemble Native American grinding holes but, in some riverbeds, form in huge numbers without any evidence of human activity.
Potholes are important because they control the speed of water in floods and because, in some cases, they are the primary mechanism by which rivers erode into bedrock. River erosion into bedrock is the primary process by which mountain ranges erode away, so these strange holes are of keen interest to geologists.
Potholes form when water flowing in a river spins in a whirlpool, picking up sediment and “drilling” into the bedrock. The sediment picked up by the water grinds against the bedrock walls of the pothole, enlarging it over time.
What has been unknown, until now, is how potholes tend to be larger in rivers with deeper floods and why they have a similar shape regardless of the depth of the flood.
The largest potholes on Earth have diameters of more than 100 feet. Such large potholes formed during the megafloods that occurred following the collapse of ice dams at the end of the last glacial epoch about 18,000 years ago. Potholes tend to be as deep as they are wide, so their shape does not change as flood depths vary from a few feet deep to the 300-foot-deep megafloods.
Working with colleagues in California and Oregon who collected field data on the sizes and shapes of potholes and the sediments that formed them, I used computer models to determine that the force of the water and sediment is greatest for potholes that are just as deep as they are wide, which matches data collected by my colleagues in the field. Potholes deeper than the ideal one-to-one relationship between diameter and depth have too much drag from the walls, and shallower potholes have too much drag from the bottom. This drag slows the water and sediment in the whirlpools before they can drill far into the bedrock.
Potholes are formed primarily by the abrasion of sediment particles that are picked up from the bottom of the pothole but not removed entirely.
Sediment particles that are too large to be picked up can’t grind away at the side of the pothole to make it bigger, and particles that are too small can’t contribute to pothole formation because they have been transported downstream.
The diameter of the “ideal” sediment particles (those that are not too big nor too small) increases with the flood depth.
Larger sediments form larger potholes partly because the pothole must be at least as big as the sediment particles that reside within them.
Computer models are useful for understanding the behavior of processes that are hard to reproduce or measure in the laboratory. Geologists often rely on computer models because many of the processes we study are difficult to reproduce or measure directly as they are happening, often because of the powerful nature of the events themselves.
Of course, computer models are just guides to what might be happening in the real world, which is always more complex than our computer models can fully represent.
As such, models must always be tested and compared against data from real-world examples.