Research has shown that movement of the tides actually cause the water to stay salty. The tide makes the sweet water reservoir move, without enabling it to increase in size. The sweet water reservoir prevents salty water flowing into the river. Damming the Hollandse IJssel could upset this balance and could move the salt line further westwards. Besides this, waste water from the city Gouda would be unable to flow into the North Sea unhindered. Not to mention the new purification plants and new sewers which would need to be installed in the city of Gouda. This is almost impossible for such an old city. A third drawback of building a closed dam in the Hollandse IJssel is a practical one. A number of shipyards, producing ships weighing up to 2000 tons each, were located along the Hollandse IJssel. For these ships, a screen lock would have to be built in the dam. However, this screen lock would not be the only one, because besides this screen lock, the dam would also need a flow screen lock for the water and a screen lock for inland navigation. This simply would not fit. Even before the disaster of 1953, it was already decided not to build a dam in the Hollandse IJssel because of the above-mentioned drawbacks.