Three rivers, the Rhine, the Meuse and the Scheldt enter the North Sea close to each other in the Netherlands, where they form the so-called delta region. This area has been under constant human influence since the Middle Ages, but especially after a catastrophic flood in 1953, when very important coastal engineering projects changed the estuarine character of the area drastically. Freshwater, brackish water and marine lakes were formed and in one of the sea arms, the Eastern Scheldt, a storm surge barrier was constructed. Only the Western Scheldt remained a true estuary. The consecutive changes in this area have been extensively monitored and an important research effort was devoted to evaluate their ecological consequences. A summary and synthesis of some of these results are presented. In particular, the stagnant marine lake Grevelingen and the consequences of the storm surge barrier in the Eastern Scheldt have received much attention. In lake Grevelingen the principal aim of the study was to develop a nitrogen model. After the lake was formed the residence time of the water increased from a few days to several years. Primary production increased and the sediments were redistributed but the primary consumers suchs as the blue mussel and cockles survived. A remarkable increase of Zostera marina beds and the snail Nassarius reticulatus was observed. The storm surge barrier in the Eastern Scheldt was just finished in 1987. Predicted consequences have been modeled and include decreased current velocity, smaller amounts of suspended material and less extinction of light and increased primary production. 'The smaller tidal volume will decrease the surface of intertidal flats available to wintering birds. This is a special concern since the delta area is of international importance for certain wader species. The Western Scheldt has not changed hydrodynamically but is polluted by industrial and organic waste. Bacterial processes involving the consecutive exhaustion of different electron acceptors, from oxygen to sulphate, are dominant in the freshwater river. There are several indications that impacts in the estuary exist as well despite the important dilution by the tides.