for National Geographic News
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-porocbig 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]."
A tidal bore can travel in excess of 20 miles an hour (30 kilometers an hour) and gets stronger as the river gets narrow and shallower, reaching recorded heights of up to 30 feet (9 meters). The pororoca, as the bores are known in the Brazilian Amazon, has been sighted over 180 miles (300 kilometers) inland on the Rio Guajara.
"Two principal factors affect the size and strength of bores," said Victor Miguel Ponce, a professor in the civil and environmental engineering department at California's San Diego State University. "The magnitude of the tidal range and the shape of the river at its estuary."
The Earth's tides are caused by the gravitational attraction of both the sun and moon on the Earth's waters. Two high tides occur every 24 hoursone on the side of our planet that is closest to the moon and the other on the side that is farthest away.
Since the sun is nearly 40 times farther from the Earth than the moon, solar tides have a smallerthough still measurableeffect. High spring tides occur whenever the Earth, moon, and sun line up and reinforce each other. These tides fall after both the new and full moon.
Weather and coastal effects also have a local and often unpredictable effect on tides. Strong winds sweeping in from the sea can cause water to pile up against the shore, leading to higher-than-expected water levels. "Extreme meteorological conditions leading to a storm surge can also produce a tidal bore in a river that may under normal conditions not exhibit one," Wright said.
But not every river has a bore, no matter how spectacular the local tides. "For a bore to occur, the shape of the river at its estuarynear its mouthhas to be shallow and uniform," with just the right water depth to accommodate the incoming wall of water, Ponce noted.
In the open ocean a tide wave is symmetrical. "As it encounters shallow water near shore, the crest [highest point] travels at a greater speed than the base, causing the wave to steepen," he said. Once the crest overtakes the trough, the peak sharpens. A discernible head wave is formed and a bore is born.
In fact, tidal bores only occur in an estimated 100 rivers in the world and only during extreme tidal ranges. A few of these bores have developed famous and sometimes deadly reputations.
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