The Merwede (etymology uncertain, possibly derived from the ancient Dutch (Merwe or Merowe) word meaning "wide water") is the name of several connected stretches of river which ultimately mouth near the cities of Dordrecht and Papendrecht, The Netherlands. The river is part of the Rhine-Meuse-Scheldt delta.
At first, the Meuse river joins the Waal at Woudrichem to form the Boven Merwede (Upper Merwede). A few kilometers downstream it splits into the Beneden Merwede (Lower Merwede) on the right and the Nieuwe Merwede (New Merwede) on the left. All these rivers are tidal. The Nieuwe Merwede joins the Bergse Maas near Lage Zwaluwe to form the Hollands Diep estuary, and separates the Island of Dordrecht from the Biesbosch National Park. The Beneden Merwede splits into the Noord River and Oude Maas near Papendrecht.
In medieval times there was no such distinction. The Merwede (or "Merwe" in Middle Dutch) was the name of a continuous stretch of river, considered to be the lower part of the river Waal (being a major distributary branch of the Rhine). Although the name Merwede is currently only used between the towns of Woudrichem and Papendrecht, during its early history it carried on his name all the way to the sea - including the rivers currently named Noord river and Nieuwe Maas.
The situation changed during the later Middle Ages, when a major flood forced the Meuse river to shift its main course towards the Merwede as well. From that moment on, several stretches of the original Merwede were named Meuse instead and served as the primary outflow of that river. These are currently known as the Oude Maas and Nieuwe Maas. However, during another series of severe floods (such as the St. Elizabeth flood) the river Meuse (and, to a lesser extent, the river Rhine as well) found an additional path towards the sea, resulting in the creation of the Biesbosch and Hollands Diep. Because of this, the influence of the river Meuse on its former estuary branches slowly diminished. In recent centuries the influence of the Meuse has decreased even further, to the point that the major stretches of river called Oude Maas and Nieuwe Maas were essentially cut off from the river Meuse completely.
From that moment on, both the current Merwede and all of its former lower stretches (containing the name Meuse or not) are only part of the delta of the Rhine, while the Meuse has been giving its own artificial mouth Bergse Maas and the two rivers Rhine and Meuse are now mostly separated to reduce the risk of flooding.