It's the tail end of the dry season on the Araguari River in...

It's the tail end of the dry season on the Araguari River in the Brazilian Amazon Basin, and the water level is low. The moon is full. Suddenly an ominous roar rolls through the jungle, like the rumble of an oncoming train. A vast wall of water comes hurtling straight up the river. The native Tupi Indians call it poroc-poroc—big roar.

Is it a tsunami? No. It's a tidal bore.

The phenomenon is "a wave that forms at the head of the incoming tide in certain rivers or estuaries," explains tidal bore surfer and researcher Tom Wright of the Tidal Bore Research Society Web site. "It is the product of the tide wave and should not be confused with a [tsunami]."

Added: January 13, 2007
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