Complex sentence: Both were connected to the Haringvliet which before the disaster was as a pure sea-water inlet of the North Sea, but after the disaster became brackish and an important estuary of the rivers Rhine and Meuse.
Split into simpler sentences: Both were connected to the Haringvliet which existed before the disaster as a pure sea-water inlet of the North Sea. After the disaster it became brackish and an important estuary of the rivers Rhine and Meuse.