forest.
Since the middle of the twentieth century, these forests have been endangered through the diking of major European rivers and reduction of the natural flooding zone, deforestation, pollution of air, water and soil, bad management of water resources and non-adapted silvicultural practices. Most of the original riparian forest has disappeared along the four major European rivers (Danube, Po, Rhine, Rhone). Forest used to cover about 2000 km2 along the Rhine. Nowadays, it is very much fragmented and covers a total of 150 km2, of which less than 1.5 km2 is still semi-natural. Only some parts of the Danube (in Hungary, Serbia-Montenegro and Romania) survive under a relatively natural regime, due to a delay in technological development. The perturbation of the flooding regime has altered the delivery of nutrients to the riparian forests. Flood water, rich in mineral particles (such as phosphates) and rainfall, is no longer filtered by soils and trees, causing eutrophication of groundwater.