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Tidal Waves, Tidal Bores and Tsunamis

Driving the Seward Highway along Turnagain Arm southeast of Anchorage one may occasionally observe a white, frothy line of turbulent water extending completely across the arm and moving slowly upstream. The column of disturbed water usually appears to be about two or three feet high and maybe ten feet across.

This is a tidal bore. It is created when rising tides in Cook Inlet encounter the constricted entrance and diminishing water depths of Turnagain Arm. Cook Inlet has exceptionally high tides to begin with, and under the "right" conditions, the tide will rise faster than Turnagain Arm can smoothly accommodate it. When it reaches a height sufficient to surmount the water already occupying the arm, it breaks over and comes rolling with a steep, high front.

Some of the other famous tidal bores in the world occur in Canada's Bay of Fundy, in the Seine River of France, and in the Amazon River.

A tidal wave, as such, is a catchall phrase connoting nothing more nor less than an unusual rise or incursion of water along the seashore. These can be attributable to a variety of factors such as offshore winds, storms or a combination of winds and spring tides. However, the term is often confused with that scourge of old movies and modern civilization alike, the tsunami.

True tsunamis (the word means "large harbor wave" in Japanese) are due to cataclysmic underwater disturbances such as submarine landslides, volcanic explosions or, most commonly, vertical displacements of the sea floor due to faulting during large earthquakes. The analogy of lifting one end of a bathtub full of water is a good one--you're bound to set a splash at the other end. If a segment of the sea floor is raised during an earthquake, the resulting "slosh" can travel across entire oceans.

Tsunamis have several remarkable features. In the open sea where the water is deep, they travel at speeds averaging about 450 miles per hour, but they would not be discernible because the wavelength is very long (usually over 100 miles) and the amplitude is very low (a foot or two).

It is only when they begin to encounter shallow water that they begin to pile up into the monstrous waves that are responsible for the destruction that is attributed to them. The Aleutian chain is probably the most prolific producer of the tsunamis in the world. In the memory of most Alaskans, the tsunami associated with the 1964 earthquake is probably the most vivid. Not only did it devastate many coastal villages and towns such as Valdez, Seward, Kodiak and Cordova (to mention but a few), but it also drowned people as far away as Crescent City, California.

However, the Aleutian-generated tsunami of this century which probably had the most profound effects outside the state occurred on April 1, 1946. This resulted from an earthquake at Unimak Island. The water at Scotch Cap on Unimak rose more than 100 feet and completely destroyed a lighthouse. In the Hawaiian Islands, more than 2000 miles to the south, waves washed to a height of 57 feet, resulting in millions of dollars of damage, and killed 159 people.