the mystery of the holes

Whenever I see any kind of natural phenomenon, I want to know how it happened, and why. A recent example has been the round holes that occur in mud banks in the local rivers, which I first noticed several months ago. The following photo, taken yesterday in the Shum Chun River on my way back from a journey to the west, shows how extensive this phenomenon is.

So how were these surprisingly regular holes in the mud of the river, each about 30cm in diameter, formed? I originally came up with two hypotheses, neither of which was even remotely convincing.

All the rivers in the northern New Territories have been canalized as a flood prevention measure, which means that the beds of the rivers were constructed using prefabricated concrete sections. And these concrete sections have neat rows of holes, each just slightly smaller than the holes in the mud. These holes remain full of water even when the river level is very low, making them ideal fishing holes for the local egrets, although it does appear, from passing observations, that this is the avian equivalent of shooting fish in a barrel. Unfortunately for my hypothesis, the holes in the mud are anything but regular in their spacing.

The second hypothesis centred on the notion of turbulence when the mud is underwater. The problem here is that it is impossible to imagine the kind of turbulence that would be required to produce so many holes, although one can imagine that the amount of water coming down the river for this to happen exceeds the design specifications of the flood defences.

It may be that the holes had always been filled with water whenever I saw them, so an obvious third hypothesis didn’t present itself: that the holes are the work of a horde of unknown creatures. This idea occurred to me only yesterday, when I saw the sandbank about 200 metres downstream from where the first photo was taken:

The holes at the water’s edge are filled with water, but those slightly higher are dry, and I was surprised to see that they are quite shallow. As can be seen in the photo, there is a small parapet of debris around each of the dry holes, which makes it highly likely that the holes were excavated. There is no sign of a burrow at the bottom of any of the holes, which would confirm the third hypothesis, so this photo is merely evidence, not proof. I suspect that crabs are the culprits, but this is just a guess at this stage.

You might wonder why I bother with such a trivial pursuit. After all, absolutely nothing of note hangs on solving this mystery. However, in an era of mobile communications, I prefer to pay attention to my environment, to listen to the sounds of nature rather than to a disembodied voice emanating from a smartphone, to look at whatever is happening in my immediate vicinity rather than at the screen of a mobile device. Unfortunately, my observations to date may have provided a mechanism for how these holes were formed, but I still don’t have any idea why. My best guess at the moment is that the holes are intended as a trap for potential prey, although I wouldn’t expect this to work if the holes are full of water.