Winter flooding in Dutch stream valley floodplains: biogeochemical effects and vegetation consequences Victor Beumer Climatic change has great impacts on stream catchments and their ecology. Expectations are that more extreme climate events will result in undesired flooding in stream catchments. In the Netherlands, former floodplains with a history of agricultural or semi natural land use are put into use again as flooding areas for the purpose of water retention. With these measures, Dutch water policy aims for more frequent, controlled flooding of stream valley floodplains to avoid unwanted flooding elsewhere. The controlled flooding usually takes place in winter in parts of valleys which had not been subject to flooding in the last decades. It may thus affect existing nature with its conservation values. This PhD-study focuses on the investigation of biogeochemical effects of surface water storage in groundwater dependent areas and its consequences for vegetation. I have considered a winter flooding event in view of three kinds of supplies, i.e. (1) water, (2) ions, and (3) organic matter. Winter flooding increases the water supply to a floodplain soil, except when saturated before flooding with groundwater or rain water. As a result, the redox potential will decrease, providing more acid neutralizing capacity through anaerobic reduction processes and toxic compounds as dissolved Fe and NH4 increase. The input of different ions is dependent on the water supply to the floodplain soil either through precipitation, groundwater exfiltration or stream water flooding. Organic matter deposited with floodwater caused an initial pH decrease, but increased the soil acid neutralization capacity as well. Organic matter that was already present in the soil before winter flooding provided CEC and soil acid neutralizing capacity. It also buffered Fe and PO4 release up to 2 weeks during flooding. Four winter flooding situations are described in the thesis and can be used to assess the direction of development of ...