Haringvliet is a large North Sea bay in the province of South Holland in the Netherlands. It is an important estuary of the Rhine and Meuse deltas.
Near Numansdorp, the Dutch Diep divides into the estuaries of the Haringvliet and the Volkerak. At Goedereede is landlocked by the Haringvlietdam, linking Voorne and Goeree-Overflakkee islands and separating it from the North Sea. The barrier with sluice gates was built as part of the construction of the Delta Works sea barrier.
In the Haringvliet estuary is the island of Tiengemeten, which is separated from the river island of Hoeksche Waard by the Vuile Gat.
The Haringvliet was formed as a result of a massive flood in 1216, which broke through the dunes of the Voorne and created a deep saltwater inlet. In the second flood, the St Elisabeth flood of 1421, the bay joined the Merwede and became an important estuary of the Rhine and Meuse.
It was once ecologically rich and its brackish waters, fed by rivers and the sea, supported a healthy seal population. It also provided access to migratory fish such as salmon and sturgeon. As a result of the North Sea floods in 1953, Haringvliet was closed by the Haringvliet Dam. Haringvliet lost its estuary and became a freshwater lake. Because of this, seals have disappeared and migrating fish can only reach the Rhine and Meuse deltas via the busy and heavily industrialised Nieuwe Waterweg.
There are plans to permanently open several of the Haringvliet locks to partially restore the ecosystem.