How do waterfalls form? Sometimes, all by themselves
Scientists discover a new mechanism for how a waterfall is born
Many of us are captivated by waterfalls. How do these cascades form? The answer, a team of geologists has found, is that sometimes waterfalls spring to life all by themselves.
Using a scale-model riverbed 24 feet long and one foot wide, the scientists have found a new mechanism that could lead to the formation of waterfalls. It doesn’t require forces such as tectonics, changes in climate, landslides or glaciers.
“We used a model river with flowing water and sediment to examine how waterfalls form,” says geologist Joel Scheingross of the University of Nevada, Reno, lead author of a paper in the journal Nature. “We wanted to see if waterfalls could form through internal feedbacks alone — the forces among water flow, sediment transport and bedrock incision.”
The experiment, says Scheingross, used synthetic bedrock made of polyurethane foam, which lacks the differences in rock characteristics known to cause waterfall formation. And like natural rock, the foam resists erosion by water alone.
Small waterfalls upstream of Bridalveil Falls in Yosemite, and waterfalls in Eaton and Rubio Canyons in the San Gabriel Mountains near Los Angeles, may have self-formed, according to Scheingross and colleagues. “The Seven Tea Cups formation along Dry Meadow Creek in California is also a great candidate for this,” says co-author Michael Lamb of the California Institute of Technology. Lamb is the project principal investigator.
Instabilities in a river’s bedrock erosion, flow and sediment transport increase the washboard characteristics of the riverbed, which can then deepen to gradually form a waterfall.
“Earth scientists have many theories for how waterfalls form under different circumstances in nature,” says Justin Lawrence, a program director in the National Science Foundation’s Division of Earth Sciences, which funded the research.
“For example, waterfalls can be produced by the intersection of rock types, changes in sea level, the thrusting of tectonic faults, climate change, landslides and glaciers,” Lawrence says. “But contrary to popular belief that there must be an external trigger, this research team found that waterfalls can form on flat riverbeds as a result of changes in flow, sediment movement and bedrock erosion within a river.”
In the experiments, Scheingross, Lamb and co-author Brian Fuller of the California Institute of Technology based hydraulics and sediment transport on mountain rivers where waterfalls are common.
In some four hours, with the model riverbed tilted at about 11 degrees, the model produced river incision and evolution. Within the first hour, the sediment formed a channel some four inches wide, with a slot canyon.
The researchers ran the experiment in 20-minute increments so they could take measurements of the bedrock erosion.
“We demonstrated that waterfalls can form in the absence of changes in climate or tectonics, due entirely to the internal dynamics of rivers — the combination of water flow hydraulics, sediment transport and bedrock erosion,” Scheingross says.
Erosion often causes waterfalls to retreat upstream after they’ve formed; falls may exist far upriver from their point of creation.
That has led some researchers to use the position of waterfalls in river networks, in combination with models for the rate at which waterfalls retreat upstream, to back-calculate the time of the climate or tectonic change that was thought to have formed the falls.
“Identifying self-formed waterfalls in nature is extremely difficult, because almost every natural landscape is subject to changes in rock type, tectonics and climate that can produce waterfalls through external forcing,” Scheingross says.
“This paper reports a proof-of-concept experiment that waterfalls can self-form. While we proposed a few examples of where waterfalls may have self-formed, more work needs to be done.”
The geologists hope their research will lead to an effort by the scientific community to tackle the problem.
“Our findings suggest that waterfalls that occur in a series in mountain landscapes are more likely to develop from the self-formation mechanism, while single waterfalls that occur in isolation, such as Niagara Falls, are not consistent with that mechanism,” says Scheingross.
Funding also came from Caltech, the Alexander von Humboldt Foundation and NASA.