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Rivers are lifelines that are of vital importance for the human society. They fulfill numerous ecological and economic functions – as habitats for the fauna and flora, as transport routes, as source for drinking and industrial water, as recreation centres and as favoured areas of settlement. Many of those utilisations are contradicting each other and all put serious pressure on the stream ecosystems. This pressure is further increased by the repeated anthropogenic intrusions that mankind uses to adjust the rivers and floodplains to its needs. Following Karr, four different kinds of intrusions can be differentiated – modifications of the habitat structures, the discharge patterns, the nutrient regime and the physicochemical water quality. Each kind of modification can effect the streams directly and indirectly.
The European rivers were constantly impacted by one or more of these intrusions throughout the course of human history, which led to manifold problems. Back in the Ancient World, rivers with their broad and easy to access gravel and sand banks were the starting points for the conquering of the mostly forested continent. This lead to an accelerated spread of plant and animal species. During the Middle Ages the enlargement of the populated area parted from the fluvial lineaments and caused extensive forest clearance, soil erosion and the sedimentation of alluvial clay. In addition to this complete alteration of the floodplains, the industrious man started to intensify the regulation of the rivers discharge with weirs and dams in order to provide a controlled water volume for energy generation, irrigation and drainage of agricultural areas and as a means of flood protection. In the course of the industrial revolution, untreated waste waters and the straightening of the rivers were added to the number of impairments, that have altered the morphological, hydraulic and physicochemical character of the larger European streams fundamentally.
Since the second half of the 20th century, the impairment of the rivers is openly discussed in Germany. Catastrophic accidents (like the Sandoz fire quench water, which contaminated the Rhine in 1986), that gained a thorough media coverage, helped to develop an awareness for the existing problems. This led to the implementation of water quality regulations and the better treatment of industrial and urban waste water.
Today, the physicochemical impairment of the German rivers is drastically reduced, which shows in an overall „good“ biological quality. The morphological degradation however is still an unsolved challenge. Hence the ecological quality of the rivers not only in Germany, but worldwide, is far away from a „good“ condition. In North America for example, more than one third of all rivers are morphological degraded or heavily polluted and the aquatic fauna is five times more prone to extinction than the terrestrial fauna.
The European Water Frame Directive (WFD) from 2000 was the first international legal framework, that aims at the ecological restoration of all European surface water bodies until 2015. In order to reach this ambitious aim, extensive river development plans have to be implemented. However, the scientific knowledge of how to improve different deficits under various constraints in a cost-effective way, is still mostly theoretical. Data about the effect of river revitalisation measures or even the most basic river structure quality is meagre, but vital for the success of the WFD. If the enormous amount of money, that has to be invested in order to reach the aims of the WFD (between 18 and 50 Bln € in Germany alone), is taken into account, the importance of closing that knowledge gap is evident.
This study deals with the morpological and faunistic aspects of two revitalsation measures in the river Lahn (Hesse, Germany) and is thus part fundamental research and part applied science.