Beneden Merwede; the Oude Maas ("Old Meuse"), which branches off from the southern branch where it changes from the Benede Merwede into the Noord, and the Dordtsche Kil, which branches off from the Oude Maas. Before the St. Elizabeth's flood (1421) the Meuse flowed just south of today's line Merwede–Oude Maas to the North Sea and formed an archipelago-like estuary with the Waal and the Lek. This system of numerous bays, estuary-like extended rivers, many islands and constant changes of the coastline, is hard to imagine today. From 1421 to 1904, the Meuse and the Waal merged further upstream at Gorinchem to form the Merwede. For flood protection reasons, the Meuse was separated from the Waal through a lock and diverted into a new outlet called the "Bergse Maas", then the Amer flowing into the former bay known as the Hollands Diep. The northwestern part of the estuary (around Hook of Holland), is still called Maasmond ("Meuse Mouth"), ignoring the fact that it now carries only water from the Rhine. This might explain the confusing naming of the various branches. The hydrography of the current delta is characterized by the delta's main arms, disconnected arms (Hollandse IJssel, Linge, Vecht, etc.) and smaller rivers and streams. Many rivers have been closed ("dammed") and now serve as drainage channels for the numerous polders. The construction of Delta Works changed the delta in the second half of the 20th century fundamentally. Currently Rhine water runs into the sea, or into former marine bays now separated from the sea, in five places, namely at the mouths of the Nieuwe Merwede, Nieuwe Waterway (Nieuwe Maas), Dordtse Kil, Spui and IJssel. The Rhine–Meuse Delta is a tidal delta, shaped not only by the sedimentation of the rivers, but also by tidal currents. This meant that high tide formed a serious risk because strong tidal currents could tear huge areas of land into the sea. Before the construction of the Delta Works, tidal influence was palpable up to Nijmegen, and even today, after the regulatory action of the Delta Works, the tide acts far inland. At the Waal, for example, the most landward tidal influence can be detected between Brakel and Zaltbommel. History Already in the time of Julius Caesar, the "Island of the Batavi" was known to the Romans. Its eastern point was the split of the Rhine into the Oude Rijn and the Waal, which