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By Leah Crane

There is more to waterfalls than we thought. We don’t have a full understanding of how they form, but some may arise all on their own, without any clear influence from the surrounding terrain.

Generally, we assume that most waterfalls form because of the features of the landscape surrounding and beneath a river. For instance, an earthquake can shove land upwards along a tectonic fault to create a cliff face over which a river cascades, or a similarly steep drop could be created by glacier movement. Alternatively, the river could simply flow over a patch of particularly easily eroded rock and gradually create a waterfall as it eats its way down through the soft bedrock.

Joel Scheingross at the University of Nevada, Reno, and his colleagues found that waterfalls can actually form without any of those factors. A river flowing downhill over smooth and homogeneous ground can develop waterfalls all on its own.

This is difficult to study in nature because of the complexity of natural landscapes. Instead, the team tested it using a tilted, 7.3-metre-long artificial riverbed made of polyurethane foam to simulate bedrock. They poured water and small pebbles into the top of this flume to make a miniature river with sediment flowing down it.

The pebbles acted like tiny chisels, beginning to erode the foam riverbed almost immediately. “Nature doesn’t like things to be flat,” says Scheingross. “Some areas get eroded a little bit more and are a little bit deeper, and others stay a little bit shallower.”

These small differences cause a feedback effect, in which pebbles flowing into the deeper areas hit the bottoms of the pools harder and create even steeper drops. Eventually, some of these steep drops become full-on waterfalls.

We often use waterfalls as proof of past climate and tectonic changes – for example, to say that a glacier carved the ground so conditions must have been cold. Self-forming waterfalls could be a problem for those assumptions.

“If it turns out, as my hunch is, that these types of waterfalls are ubiquitous, then I think it will change how we’ve interpreted some past changes in climate and tectonics,” says Scheingross. First, though, we’ll have to find self-forming waterfalls in the wild.

Journal reference: Nature, DOI: 10.1038/s41586-019-0991-z