Ecological restoration of free flowing and impounded stretches of the Austrian Danube River – towards the objectives of the Water Framework Directive | Ökologische Sanierung von Fließstrecken und Stauhaltungen der österreichischen Donau – auf dem Weg zur Zielerreichung nach Wasserrahmenrichtlinie

Concerning the Austrian Danube River, hydromorphological pressures are the main shortfalls which prohibit achieving an ecological status as prescribed by the EU Water Framework Directive. The relevant biological quality element for evaluating this type of alteration is the fish fauna. Its generally low stocks – especially of rheophilic, type-specific species like nase (Chondrostoma nasus) and barbel (Barbus barbus) – mostly pose the main criterion relevant to the evaluation of the overall ecological status. Therefore, rehabilitation measures have to focus on providing and improving type-specific river habitats and, above all, key habitats like spawning and nursery grounds for the rheophilic Danube fish fauna.For the last three decades, many rehabilitation measures along the Austrian Danube were put into practice and their effect on the fish fauna is well documented. Based on the case studies Wachau (free flowing stretch) as well as Engelhartszell and Wilhering (heads of impoundments) the effective rehabilitation measures for the consolidation of reophilic key species are deduced. Therefore the following restoration types can be considered as most promising for achieving the objectives of the Water Framework Directive: (1) Concerning head of impoundments in narrow Danube valleys only the utilization of the full potential for instream and riparian structures through spreading of gravel banks or riverbank restoration through removing embankment protection is viable. (2) In free flowing stretches as well as head of impoundments in broad Danube valleys the construction/reactivation of side branches and gravel islands with extensive and sustainable shallow bank areas is possible as well. (3) Furthermore the construction of bypass channels with sufficient flow dynamics (!), especially in combination with subsequent side branches and/or instream structures is considered highly effective. (4) Last but not least preventing the deepening of the river bottom by bed load management is a basic prerequisite for the enduring success of restoration measures and the ecological integrity of the river ecosystem as a whole.Within the Austrian Danube River the relevant fish species composition for evaluating the ecological status in terms of the Water Framework Directive is present in all different river stretches, even though these are divided by dams. The interconnection of those severed river stretches by creating fish passes may be important in the medium and long term to preserve sound fish populations. Furthermore, re-establishing the lateral connectivity to backwaters and tributaries can provide access to important key habitats. Whereas the contribution of longitudinal connectivity to the objectives of the Water Framework Directive within impounded stretches of the Austrian Danube River in poor or bad ecological status/potential must be considered as low. Technical fish passes can hardly contribute to the restoration of main ecological deficits such as the lack of suitable habitats for rheophilic fish. Without comprehensive rehabilitation measures the targets of the Water Framework Directive cannot be met. In case of technical feasibility, longitudinal connectivity measures should therefore be combined synergetically with habitat improvement by construction of nature like bypass channels with natural flow dynamics and suitable spawning and nursery habitats for rheophilic fish species. The implementation of such rehabilitation measures can provide significant contributions to the objectives of the Water Framework Directive as well as the objectives of the Habitats Directive.