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Rainfall influences how quickly rivers cut through rock

by CIFAR May 2 / 13

Throughout Earth’s history, rivers have shaped the structure of mountains and transformed Earth’s environment.

The Hanalei River on Kaua‘i. The river helped shape the mountains in the background.
CREDIT: Taylor Perron.

Scientists have long suspected that heavier rainfall causes rivers to cut through rock more quickly, resulting in a diversity of landscapes on Earth. Researchers have studied this in different parts of the world, but have had difficulty reaching a consensus on the net effect of rainfall rates on erosion rates. This is in part because different regions have different rock types, vegetation, human activity and tectonic history, each of which affects erosion rates.

Now, Senior Fellow Taylor Perron (MIT), Global Scholar Ken Ferrier (Harvard), and their colleague Kimberly Huppert (MIT) have focused on the Hawaiian island of Kaua‘i and present clear evidence that rivers cut through rock faster where it’s wetter. Their findings were published in Nature.

“Kaua‘i is a remarkable natural experiment because it has one of the world’s most striking rainfall gradients, with average annual rainfall ranging from 0.5 metres to 9.5 metres – over 70% of the global range” says Ferrier. “And in contrast to other places around the world, non-climatic factors that affect erosion, like rock type and tectonic history, have little variability across the island. All of these advantages have made Kaua‘i an excellent natural laboratory for exploring the effect of rainfall on river erosion.”

The team used a mathematical model that calculated stream power – the rate at which a river expends energy upon the riverbed – in a number of rivers on Kaua‘i. Places with higher rainfall rates have higher fluxes of water through the river, and hence higher steam power. By comparing estimates of stream power with the team’s measurements of river erosion, the researchers were able to show that higher rainfall translated into greater river erosion.

“The river erosion rates we measured in this study are averages over the past 4-5 million years,” says Ferrier. “We are now measuring more recent erosion rates around Kaua‘i and expect new results soon. Looking at modern erosion rates is important because the sediment that rivers currently discharge into the ocean can strongly affect the health of the coral reefs offshore. Studying current erosion rates will inform options for managing the rate of sediment washing into coastal ecosystems.”

The findings from this study also provide a foundation for understanding the evolution of the Earth’s surface, including some of Earth’s most dramatic landscapes, from towering mountain ranges to deeply dissected tropical islands.

This study was supported by the Massachusetts Institute of Technology.