Source: Scheingross, Joel S., Michael P. Lamb, Brian M. Fuller. Self-formed bedrock waterfalls. Nature, 567:229-233. https://doi.org/10.1038/s41586-019-0991-z.
It’s summer in the Northern Hemisphere, and people are flocking to their local swimming holes for a reprieve from the heat. For some, these local river destinations will echo with the delighted sounds of bathers and the laughing crash of falling water.
Falling water captures the imagination. Some may even pause to consider, “Why do waterfalls form?”
For geologists and historians, waterfalls are often considered evidence of historical change in the landscape. They can take thousands of years to form. Changes that lead to their formation include, for example, a retreating glacier, a change in total rainfall, sea-level drop, or when bedrock is pushed up along a tectonic fault line, like in an earthquake. Using these and other clues, geologists can reconstruct the historical story of the land, climate, and even the earth itself.
Now, a third way waterfalls form has been revealed.
Waterfalls that form in uniform bedrock formations may do so not as a result of change, but by random chance.
Joel Scheingross and his colleagues at the California Institute of Technology demonstrate this in the lab.
Experiment & Findings
To simulate the conditions that might lead to waterfall formation, Scheingross and his team started with a uniform foam material that behaves similar to natural bedrock. They tilted the foam to a slope of 19.5%, then poured water with sand down the chute. The sand was intended to simulate the dirt and debris of a natural river system that experiences erosion and turbulent re-suspension of sediment. They stopped the experiment periodically to measure the shape of the river channel as it changed over time.
When the force of the water made the sand material crash into the foam bedrock it would scour away small depressions. The depressions would then be further scratched or scoured out by more water and debris. Sometimes material would become trapped in these depressions and stick around, growing or protecting the further development of the pools. After 2.1 hours a cascade of waterfalls had started to form in the experiment. Fully formed waterfalls lasted for about 20 minutes in the experiment. This is equal to between 100 and 1000 years in a natural river system
There are several examples from the real world, like The Seven Teacups waterfall on the Dry Meadow Creek in California, USA (shown above). This work has the potential to make us revisit what we know about how water falls form, and what they are telling us about the history of the land. Luckily, we tend to like revisiting waterfalls… especially on a hot day!
Cover image available at: http://ropewiki.com/File:Seven_Teacups_Banner.jpg