The Biesbosch was created when 300 square kilometres of polder lands were submerged in the St. Elizabeth flood in the year 1421.
Before this, the area was called Grote Hollandse Waard, containing cultivated land and a number of villages.
The more than a century old dikes collapsed because of lack of maintenance, due to the difficult economic situation in the area, and the difficulties between the political entities within (especially the Hook and Cod (civil) wars).

One of the key factors in the flooding of the Grote Waard was the creation of a new dike in the southwest of the polder.
The ground beneath this dike was unstable, which was a known (but underestimated) fact in this period of time.
However, combined with political rivalry and financial issues, this resulted in the creation of an unstable dike, located at one of the key corners of the Grote Waard.
It was the only place where high tides coming directly from the sea could penetrate the land deeply and could reach a primary "keystone" dike of the Grote Waard (most of the primary dikes were used as protection against rivers instead).

Very high river levels combined with a severe storm surge coming in from the sea caused the collapse of the southwest dike and several riverdikes, resulting in the flooding of most of the Grote Hollandse Waard.
After the flood, three areas remained: the Island of Dordrecht to the west, the Land van Altena (with the city of Woudrichem) to the east, and the brackish swamps of the Biesbosch in between.
Many villages (by tradition 72) were swallowed by the flood and were lost, causing (again by tradition) either 2,000 or 10,000 casualties.
Some towns had more luck and submerged, only to rise above the water later on.

Many Rhine-Meuse delta branches that were closed off from the main rivers and served as drainage bodies of the Grote Waard, also disappeared.
These include the Dubbel (the name of which survives in Dubbeldam), Eem, Werken (after which Werkendam is named), Graaf, Alm (which lent its name to Almkerk; the eastern part survived) and most of the main drainage stream: the Oude Maas (a small part survives in the form of the Oude Maasje, not to be confused with another branch called Oude Maas near Rotterdam).

Most of the flooded area changed into a network of estuaries.
The most important of those were the Hollands Diep ("Holland Deep") and the (shallow) Heidezee ("Heath Sea").
Both were connected to the Haringvliet which existed before the disaster as a pure seawater inlet of the North Sea.
After the disaster it became brackish and an important estuary of the rivers Rhine and Meuse.
A persistent misunderstanding is that the Biesbosch arose by this storm flood in one night.
It is true that this flood broke dikes of the then Grote Hollandse Waard or Zuid Hollandse Waard, but it needed dozens of years before the whole area was under water and had changed to the Biesbosch with its creeks and reeds.
The Biesbosch was created when 300 square kilometres of polder lands were submerged in the St. Elizabeth flood in the year 1421. Before this, the area was called Grote Hollandse Waard, containing cultivated land and a number of villages. The more than a century old dikes collapsed because of lack of maintenance, due to the difficult economic situation in the area, and the difficulties between the political entities within (especially the Hook and Cod (civil) wars).
One of the key factors in the flooding of the Grote Waard was the creation of a new dike in the southwest of the polder. The ground beneath this dike was unstable, which was a known (but underestimated) fact in this period of time. However, combined with political rivalry and financial issues, this resulted in the creation of an unstable dike, located at one of the key corners of the Grote Waard. It was the only place where high tides coming directly from the sea could penetrate the land deeply and could reach a primary "keystone" dike of the Grote Waard (most of the primary dikes were used as protection against rivers instead).
Very high river levels combined with a severe storm surge coming in from the sea caused the collapse of the southwest dike and several riverdikes, resulting in the flooding of most of the Grote Hollandse Waard. After the flood, three areas remained: the Island of Dordrecht to the west, the Land van Altena (with the city of Woudrichem) to the east, and the brackish swamps of the Biesbosch in between. Many villages (by tradition 72) were swallowed by the flood and were lost, causing (again by tradition) either 2,000 or 10,000 casualties. Some towns had more luck and submerged, only to rise above the water later on.
Many Rhine-Meuse delta branches that were closed off from the main rivers and served as drainage bodies of the Grote Waard, also disappeared. These include the Dubbel (the name of which survives in Dubbeldam), Eem, Werken (after which Werkendam is named), Graaf, Alm (which lent its name to Almkerk; the eastern part survived) and most of the main drainage stream: the Oude Maas (a small part survives in the form of the Oude Maasje, not to be confused with another branch called Oude Maas near Rotterdam).
Most of the flooded area changed into a network of estuaries. The most important of those were the Hollands Diep ("Holland Deep") and the (shallow) Heidezee ("Heath Sea"). Both were connected to the Haringvliet which existed before the disaster as a pure seawater inlet of the North Sea. After the disaster it became brackish and an important estuary of the rivers Rhine and Meuse. A persistent misunderstanding is that the Biesbosch arose by this storm flood in one night. It is true that this flood broke dikes of the then Grote Hollandse Waard or Zuid Hollandse Waard, but it needed dozens of years before the whole area was under water and had changed to the Biesbosch with its creeks and reeds.
Biesbosch National Park