River catchment area and river basin district
The river catchment area and the river basin district are superordinate hydrological terms.
The river catchment area covers a river from its source to its mouth in the sea. This also includes all its tributaries and the groundwater that occurs in this area.
The Member States of the European Union determine the individual catchment areas within their respective territories and assign them to the international river basin districts (IRBDs). They consist of one or more neighbouring catchment areas.
These serve as the basis for large-scale planning, which must also be internationally harmonised and coordinated, and are thus the main units for the management of catchment areas.
The EU and its Member States have divided the river basins and their associated coastal areas into 110 river basin districts. 40 of them have a cross-border character.
Austria has a share in three international river basin districts (RBD)
The largest and most important international river basin district to which Austria belongs is the "Danube". It includes partial areas of 19 countries! 80,593 km² of this 801,463 km² long unit are located on Austrian territory, which corresponds to 96.1% of Austria's surface area.
The Danube itself is around 2,780 kilometres long, while the length of the stretch in Austria is around 357 kilometres. The average annual discharge at the mouth of the Danube is 6,500 m³/s. The most important tributaries of the Danube are the Inn, the Drau, the March and - outside Austrian territory - the Sava, the Tisza and the Sereth.
The second, significantly smaller, unit is the "Rhine". Sub-areas of nine states have a share in this RBD. The area covers a total of 185,800 km². Austrian territory accounts for 2,366 km², i.e. 2.8% of the national territory. The most important tributaries of the Rhine lie outside Austria and are the Aare, the Moselle, the Main, the Neckar, the Lahn, the Ruhr and the ILL.
The third RBD, in which Austria has a share, is the "Elbe". Sub-areas from four countries belong to it. It covers a total area of 148,268 km². The Austrian share of the area is only 920 km², i.e. 1.1% of the area of Austria. The Elbe itself does not have a flow section in Austria. Only feeders are located here.