The Euripus is the name given to the narrow stretch of water which separates the land mass of Euboea from the mainland of Greece at its closest point, where it is less than forty metres from the Boeotian coast. Islands always form barriers to the tidal movement of waters, but the effect is generally not immediately apparent unless that barrier is particularly long (e.g Jura in the Hebrides) and/or confines a narrow area of water against the mainland (e.g the Isle of Wight). Euboea falls egregiously into both categories, and its considerable length means that the (albeit small) tidal fluctuations up the Euboean channel from the south end, and those down the channel from around the northern end of the island are separated by several hours. The resultant effect at the narrowest point of the Euripus is therefore highly anomalous, giving rise to alternating currents, which can change direction as often as six or seven times a day. The current flows from north to south for about three hours at a rate which can vary between 6 and 12 knots. It then suddenly subsides; and, after a few minutes of quiescence, it begins to flow again in the opposite direction. These currents are driven by the gradient which forms between the respective water-levels to either side of the narrows, caused by the restriction of tidal movement at the bottleneck in the strait. Tides in the Mediterranean are weak by comparison with those in the open ocean; but the considerable fluctuations in the depth of the sea-bed in the Euripus (an unusual element which it has in common with the treacherous straits of Corrievreckan, off Jura), as well as the constriction of the channel help magnify their effect disproportionately at this point.
The exact mechanics and timing of the water flow are still not fully understood. Its behaviour was widely speculated on from ancient times. Socrates (Phaedo, 90) uses the variability of the Euripus as a metaphor for that which is in a constant state of flux. The phenomenon is alluded to by Aeschylus (Agamemnon 190), as well as by Livy, Cicero, Pliny and Strabo. According to a frivolous popular tradition, Aristotle, in despair at his failure adequately to explain the phenomenon, is said to have flung himself into the Euripus. Passage through the channel with the current can be dangerous and the bridge is opened only when the flow is favourable. The capricious narrows were first spanned in 411 BC by what apears to have been a wooden bridge. In 334 BC, Chalcis included the Boeotian fort of Kanethos (across the channel) within its city boundaries. In the 6th century AD, under Justinian, the fixed bridge was replaced by a movable structure in order to facilitate the movement of vessels. The Turks then replaced this with another fixed bridge in the 15th century. In 1856 a wooden swing bridge was erected; this was superseded by the first iron swing bridge, built in 1896 by a Belgian company who enlarged the channel and demolished the Venetian fort that had guarded the approach. This gave place in 1962 to the existing structure. The new road bridge, considerably further south, was opened in 1993.
Info: McGilchrist's Greek Islands
(From McGilchrist’s Greek Islands, © Nigel McGilchrist 2010, excerpted with his gracious permission. Click for the books)