The Mysterious Tides of Evripos
BY AWAKE! WRITER IN GREECE
ON THE eastern side of Greece, near the city of Khalkís, a narrow strait divides the mainland from the island of Évvoia. This is the channel of Evripos. It measures five miles [8 km] long and varies in width from one mile [1.6 km] to only 130 feet [40 m]. At its shallowest point, it is only 20 feet [6 m] deep. The name Evripos, meaning “Swift Current,” well describes the sometimes violent flow of the channel’s waters, which often attain a speed of nearly 14 miles [20 km] per hour. Strangely, however, on some days the current seems to vacillate weakly, or it may even stop altogether! Many visitors to Khalkís come to a small bridge over the channel to observe this unusual tidal phenomenon.
Tides occur because the sun and the moon pull on the earth’s seas. For this reason, tides change according to the earth’s position in relation to the sun and the moon. During the time of the new moon, the sun and the moon lie on the same side of the earth. During the full moon, they are on opposite sides of the earth. In both cases, the sun and the moon work together, producing the highest tides.
The Evripos channel usually experiences two high tides and two low tides about every 24 hours. The current flows in one direction for 6 hours and 13 minutes, pauses briefly, and then reverses itself and flows in the opposite direction. It follows this regular pattern for 23 or 24 days of the lunar month. However, during the final four or five days of the month, unusual things happen. On some days the current may not change at all. On others, it may reverse itself as many as 14 times!
Attempts to Explain the Phenomenon
The phenomenon of Evripos has perplexed observers for thousands of years. Popular tradition has it that Aristotle, of the fourth century B.C.E., drowned here when he threw himself into the channel in despair over not solving the riddle of the tides. In reality, rather than drown himself, he attempted an explanation of the tides. In his work Meteorologica, he wrote: “It seems as if the sea flows through the narrow gap because of the surrounding land. It flows from a smaller body of water into a larger body because of the oscillating of the ground.” Aristotle mistakenly thought that the ground itself swayed because of the waves of the sea and because of the earthquakes that are prevalent in the area. About a century later, Greek astronomer Eratosthenes recognized that “on each side [of the channel] the sea has a different level.” He thought that the currents occurred because the two banks of the strait differed in height.
Even today, the irregularity of the tides of Evripos is not fully understood. But it seems clear that the regular current results from a difference in water level at the two ends of the channel. This causes the water to rush down from the higher level to the lower level. The difference can be as much as 16 inches [40 cm], and it is visible from the bridge at Khalkís.
Why the Difference?
How can the difference in water level be explained? The incoming tidal stream from the eastern Mediterranean splits into two branches when it reaches Évvoia Island. The western branch flows into the southern end of the channel. However, the eastern branch must travel all around the island before entering the channel from the north. This longer route delays the eastern stream’s arrival at Evripos by about an hour and a quarter. Under these circumstances, the sea level and consequently the water pressure on one side of the channel can be much greater than on the other side. The added pressure amplifies the force of the regular tidal currents flowing through Evripos.
But what about the irregular currents? During the first and the last quarters of the moon, the gravity of the sun works against the moon’s gravity instead of reinforcing it. While the moon is producing a low tide, the pull of the sun is working toward causing a high tide. As a result, at these times there is less difference between the sea levels in the northern and the southern ends of the channel. Thus, the force of the current is diminished. Sometimes, when the wind gets involved, the current is completely neutralized and comes to a halt.
There is, of course, even more that could be told about the current’s intriguing, mysterious behavior. If you are ever in Greece, come to Évvoia and observe for yourself the fascinating phenomenon of the tides of Evripos!
[Maps on page 12, 13]
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The Strait of Evripos
Mountain High Maps® Copyright © 1997 Digital Wisdom, Inc.
[Picture on page 13]
Aerial photo of the Strait of Evripos