Most waterfalls tell a clear story about their origins: Yosemite Falls in California cascades over a sheer granite cliff, the remains of ice age glaciers that once carved out the valley’s steep walls. Others are the result of major earthquakes or sudden changes in rock type. But some, like the Seven Teacups in California’s Sierra Nevada mountains (above), have no obvious external cause. Now, scientists have a new potential explanation for these mysterious waterfalls.
What makes the Seven Teacups so inscrutable is the unbroken granite slope they’re etched into. There’s no sudden cliff to suggest that past earthquakes or retreating glaciers created the falls, and the composition of the granite is roughly the same throughout, meaning a change from harder to softer rock wasn’t responsible for the falls’ creation.
To test whether such a waterfall could form without external causes, researchers built a 7.3-meter artificial river, or flume, with a 20% downhill slope. For 20 minutes at a time, researchers sent a steady flow of gravel-laden water along a flat riverbed made of soft foam. Then, the team used a laser scanner to measure any changes in the surface of the foam riverbed.
Over the course of 11 trials, the gravel and water wore away tiny undulations in the foam. Two of those depressions got deeper and deeper until they became miniature, selfmade waterfalls, the researchers report today in Nature. Feedback from the water, the gravel, and the erosion of the artificial riverbed interacted to make the cascades, suggesting the same mechanism could be responsible for waterfalls like the Seven Teacups.
To determine how widespread this phenomenon is, the researchers say they’ll need to chase more waterfalls—and develop some way to identify these selfmade cascades. The findings could clarify the origins of such waterfalls on Earth and even Mars, where astronomers have spotted dried-up riverbeds—and what may be the remains of martian waterfalls. If waterfalls can arise without external causes, scientists will need to be cautious in using them to reverse engineer the Red Planet’s history.