When it comes to creating waterfalls, some rivers are opting for the “do-it-yourself” approach. A study published today in Nature suggests that for some rivers, natural streamflow mechanisms can form waterfalls without any external factors. The conditions that lead to these autogenic waterfalls might be common in mountain runoff streams and should be further studied, the researchers said.
“Identifying autogenic waterfalls in the field is quite tricky,” said Joel Scheingross, a geoscientist at the University of Nevada, Reno, who led the study. The team set up a scaled-down synthetic river in a lab to study autogenic waterfalls in isolation. “Experiments allow us to control for constant rock type, water discharge, sediment supply, et cetera, and let us test the autogenic formation mechanism in the simplest possible scenario without the added complications that exist in nature,” he said.
Externally forced waterfalls can form when water flows from one rock type to the next, glaciers retreat, landslides reroute a river, or tectonics abruptly shifts the landscape, for example. Past laboratory studies of waterfall formation have simulated one or more of these factors, the team said, or used materials that erode with water alone without the presence of sediment.
In this study, Scheingross and his team set up a synthetic riverbed made from polyurethane foam to mimic a uniform bedrock, and they flowed water and sediment down its surface. They chose the experiment parameters to mimic mountain streams where waterfalls are common. The test lasted just under 4 hours, but when the model is scaled up, it represents 100–10,000 years of river evolution.
The researchers found that the flow carved out a series of repeating steps in the bedrock, similar to past experiments. Some steps gradually migrated downstream, like an escalator, as the water eroded rock from the edges of the steps.
Occasionally, however, a step built up a deposit of gravel that protected it from rapid erosion. The step below it continued to abrade and eventually became a drop steep enough to form a free-flowing jet and a plunge pool beneath—an autogenic waterfall. The team found that autogenic waterfalls tended to form a series running along the length of a river, and each fall lasted about 20 minutes (10–10,000 years in nature).
The team is conducting further tests to explore the range of conditions that might form an autogenic waterfall.
“If we can form autogenic waterfalls in many different experiments, we can also explore [whether] there’s a morphologic ‘fingerprint’ of autogenic waterfalls,” Scheingross said, like the ratio of different step heights.
Naturally Formed, Found in Nature
The researchers point out two regions in California that might have autogenic waterfalls: the central Sierra Madre block of the San Gabriel Mountains and Bridalveil Creek in Yosemite. Both sites have a series of waterfalls, something predicted by this experiment.
The San Gabriel Mountain site is “intriguing” Scheingross said, because “erosion rates and uplift rates have been in balance over decadal to million-year timescales, so it seems unlikely that tectonics could be causing waterfall development.” And while Yosemite’s famous Bridalveil Fall formed from glacial carving, smaller falls upstream might be autogenic, he said.
This study is a “proof of concept,” Scheingross said. “The hope is that once we can conclusively distinguish between self-formed and externally forced waterfalls in nature, we can then start to more critically evaluate how to use waterfalls and knickzones in interpreting Earth history.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer