Janauer, Georg A.
The Danube (length: 2,850 km, catchment: 805,000 km2) is an exceptional European river: it flows through ten countries (Germany, Austria, Slovakia, Hungary, Croatia, Serbia & Montenegro, Romania, Bulgaria, Moldavia, Ukraine) and seven more belong to its catchment (Switzerland, Italy, Czech Republic, Poland, Bosnia-Herzegovina, Albania, Slovenia). It is the second largest river in Europe, but the mean total annual discharge is only 13 % less than that of the much longer Volga. The headstreams Breg and Brigach create the Danube in Germany. Most of the water originates from the Alpine tributaries Isar, Inn, Traun, and Enns in the upper reaches of the river, the Drava and Sava flowing from the Southern Alps, Tisza from the Carpathian Ridge. The Siret and Prut in the lower reaches are other main tributaries. Therefore a study of the aquatic vegetation of this large river is of prime scientific interest. The fluvial corridor of the Danube is an ecosystem of high structural diversity. It combines confined and wide reaches of the main channel with branches of variable size in the geomorphological basins and numerous types of active and relict backwaters in the flood plains. With this abiotic diversity of structure its native aquatic plants are present in many different types of water body. In many instances rare and endangered species have found shelter, when land use changes and increasing intensity of agricultural practices eliminated suitable biotopes in the adjacent landscape and degraded rivers and backwaters.