 The European Water Divide applies only for those waters that pass beyond this point, and only during the days of the year when the Danube carries enough water to survive the sink holes in the Donauversickerung.Since such large volumes of underground water erode much of the surrounding limestone, it is estimated that the Danube upper course will one day disappear entirely in favor of the Rhine, an event called stream capturing.The hydrological parameters of Danube are regularly monitored in Croatia at Batina, Dalj, Vukovar and Ilok.