Sept. 14, 2007 — When typhoon Mindulle flooded western Taiwan in 2004, it sent the equivalent of half of the Mississippi River's annual sediment load into the Taiwan Strait via the Choshui River in just three days. Surprising new research shows that the incredible 70 million tons of mud then disappeared from the offshore seafloor in just a few weeks.
Oceanographers and geologists are only now beginning to understand the causes of Taiwan's dark deluges and the secrets of its offshore mega mud-moving power.
Among the lessons they are learning is that even short rivers can play major roles in moving earth and shaping the land and seafloors.
"The Mississippi has been nicknamed the 'Big Muddy,'" said oceanographer John Milliman of the College of William and Mary in Virginia. But the average amount of sediment suspended in the Mississippi's waters is about half a gram per liter, he said. Taiwanese rivers, on the other hand, can carry 500 grams of muck and sand per liter.
"That's a thousand times the Mississippi River's load," he said.
When river water carries more than 40 grams of sand and silt per liter, the flow is called "hyperpycnal," Milliman explained. But it's more than just a fancy word for muddy water. When hyperpycnal waters reach the sea they do something other freshwater river waters do not: They sink.
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This is exactly the opposite of what usually happens when freshwater flows into saltwater, which is usually denser.
Taiwan is the world's leader in hyperpycnal flows, Milliman said. The reason is simple: Lots of landslides and lots of torrential rain from being located in what meteorologists call Typhoon Alley.
"There are landslides everywhere," said Milliman. "Then they get heavy rains on this nicely exposed material."
