Verteilung, Diversität und Zustand von Inseln entlang europäischer Flussläufe
Islands are important elements of dynamic riverine landscapes. Islands are surrounded by stream channels and / or bare sediments and covered by perennial vegetation. They may occur in any part of the drainage network, but are most common in braided reaches of large floodplain rivers and in coastal deltas. Islands provide high numbers of habitats for aquatic and terrestrial species. Therefore, island-braided river reaches show the highest biodiversity within the river continuum.
For island building, a natural flow regime with periodic floods, a constant supply of sediment, sources of dead and living wood, and space are needed. Worldwide, but especially in Europe, few rivers remain in this natural state. River fragmentation by dams, straightening and embanking, and intensive land use in floodplains led to vast changes in hydrology, morphology and ecology of most European river systems. As a consequence, European rivers are almost devoid of islands. Due to their sensitivity to alterations of natural their regimes, islands are considered as indicators for the ecological integrity of river systems.
This study analyses the distribution (density), diversity and state of islands along 12 European rivers. Factors, which affect these parameters, were identified.
Based on 72 Landsat-ETM+-scenes, the rivers Volga, Oka, Varzuga, Danube, Sava, Olt, Siret, Prut, Velika Morava, Tagliamento, Rhine and Ebro have been mapped. Location, area and perimeter of river channel and islands have been calculated. State of islands and floodplains around islands has been analysed.
2’771 islands have been found, with individual areas from 0.06 ha to 9’828 ha. Density of islands is from 0.06 to 3.95 islands per 100 ha active river channel, and from 0.07 to 1.6 islands per river kilometer. Up to 21% of active river channel is found to be covered by islands. Furthermore, islands contribute up to 32% to total riparian ecotone length of rivers. Highest values are found in small, moderate to unfragmented rivers in catchments with moderate land use.
93% of islands are in a near natural state. 5% are used for agriculture, and 2% are urbanised. In contrast, more than 50% of floodplains adjacent to islands are intensively used for agriculture or are urbanised. Only 7% of islands lie within a natural floodplain landscape.
Comparisons with historical maps show that up to 93% of islands are disappeared due to increasing river work and dam building.
Maintaining and enhancing island building, and preserving existing islands, are important for improving ecological integrity of river systems. To reach this, adjustments in river work, floodplain management and dam operation should be made, especially concerning flow and sediment regime.