The Afgedamde Maas is a former distributary of the Maas River (French: Meuse) in the Dutchprovince of North Brabant. The Maas splits near Heusden with one channel flowing north until its confluence with the river Waal (the main distributary of the river Rhine) to form the Merwede, while the other (the Bergse Maas) continues west as the main distributary of the Maas. The former of the two channels was dammed at Heusden in 1904, to separate the flows of the Maas and Rhine distributaries, and has since been known as the Afgedamde Maas (literally, "dammed-up"). However, the Maas is still connected with the Afgedamde Maas through the Heusden Canal. Part of the Merwede (the Nieuwe Merwede) joins the Bergse Maas to from the Hollands Diep estuary, thus eventually re-uniting the two branches of the Maas.
The distribution of the Maas changed during the later Middle Ages, when a major flood forced it to shift its main course towards the Merwede river. From then on, several stretches of the original Merwede were named Maas 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 the Maas found an additional path towards the sea, resulting in the creation of the Biesbosch and Hollands Diep. The branch leading to the Merwede is the current Afgedamde Maas, whilst the other branch of the Maas eventually silted up and now forms a stream called Oude Maasje.
The canalised Bergse Maas, which takes its name from the town of Geertruidenberg, was constructed in the basin of the latter branch, to take over its functions, in 1904. At the same time, the Afgedamde Maas was dammed up, so that the flow of Maas is effectively separated from that of the Rhine distributaries. The resulting separation of the rivers Rhine and Meuse is considered to be the greatest achievement in Dutch hydraulic engineering before the completion of the Zuiderzee Works and Delta Works.