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The IJ (pronounced [ɛi̯]; sometimes shown on old maps as Y or Ye) is a body of water, formerly a bay, in the Dutch province of North Holland. It is known for being Amsterdam's waterfront. Its name is an obsolete Dutch word meaning "water", derived from the West Frisian word ie "stream, small river" (from Germanic , "water") and is cognate with Dutch for bodies of water. The name consists of the digraph ij, which behaves like a single letter. Therefore, both letters are capitalized; cf. IJmuiden.

Today, the IJ is divided into two parts:

The IJ is connected to the North Sea to the west and the IJmeer to the east by a set of locks.

There are several theories about the origins of the IJ. Perhaps it began as a stream, following a breakthrough in the dunes of Castricum. More likely, the IJ is a remnant of a northern arm of the Rhine delta. Finally, the IJ could also come from the lake Almere or Flevo. During the Roman period the IJ communicated on one side with the lake Flevo and the Vecht (Utrecht) and the other with the North Sea. Connection with the North Sea has subsequently disappeared, while the IJ in the Middle Ages has expanded. This is due to the emergence of the Zuiderzee, itself a bay of the North Sea resulting from a number of storms.

At the end of the Middle Ages, the IJ was a long and narrow brackish bay that connected to the Zuiderzee and stretched from Amsterdam in the east to Velsen in the west. At its west end, only the natural dune ridge across the Dutch North Sea coast prevented the IJ, which grew ever larger through the centuries, from directly connecting to the North Sea and so making the North Holland peninsula nearly an island. By the seventeenth century, however, access to the IJ became difficult due to sand bars across its mouth, and ships becoming bigger, and it was nearly impossible for seafaring vessels to reach the city of Amsterdam. At the same time, the bay gnawed away at the surrounding farmlands, almost connecting with the Haarlemmermeer (Lake Haarlem) and seriously threatening the cities of Haarlem and Amsterdam.

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Wikipedia