For the impact assessment of the Gabčíkovo/Nagymaros Barrage System, a monitoring program was started in the flood plain forests of the Szigetköz (N-W Hungary) in 1986. Since then, annual growth of stands has been measured in 35 permanent plots, weekly girth growth of selected trees in these plots has been taken, and health of selected trees on 60 plots has been assessed by field observations. After the completion of the Gabčíkovo dam, the Danube was diverted along a section of almost 40 km in October 1992. In 1995, a water recharge system was put into operation, but no floods have ever been experienced since 1992. As a consequence, tree growth dropped in many places at varying rates shortly after the diversion. Damaged trees appeared mainly along the main riverbed and trees lost their leaves abnormally early. Girth growth pattern became abnormal within the vegetation season, too. The adverse phenomena can only be reversed if an effective recharge system is put into operation, the principles of which are detailed in the paper.