Rhine water flows into the North Sea at three locations: in the Haringvliet, passing by the Nieuwe Waterweg and through the Ijssel Sea. The southern main branch (Noord and Nieuwe Maas) flows through the Nieuwe Waterweg into the North Sea west of Rotterdam. From this main branch the Nieuwe Merwede branches off south and flows into the Hollands Diep at Dordrecht. This former bay of the North Sea continues into the Haringvliet which developed when, in 1216, a storm tide tore a gap into the dunes.
Between 1958 and 1970 and following the Delta Plan, a dam was constructed, cutting the Haringvliet off the sea. Floodgates allow navigation. In 2004, it was decided to partially open the floodgates in order to allow for a continuous flow of salt water, to allow fish migration and to restore the brackish water region up to the line Middelharnis – Spui. This measure equally aims at renaturing the Biesbosch, a marshland near the Hollands Diep which used to be influenced by tides.
It will presumably be possible to open the sluices in December 2018.
The Ijssel flows into the Ijssel Sea. The construction of dikes turned the former Zuiderzee bay into a 1100 km² freshwater lake (double the size of Lake Constance). At the same time, due to land reclamation (construction of dikes) the province of Flevoland developed. The Ijssel Sea is a popular holiday and sailing resort.