Leitbild-specific measures for the rehabilitation of the heavily modified Austrian Danube River.
Jungwirth, Mathias; Haidvogl, G.; Hohensinner, S.; Muhar, S.; Schmutz, S.; Waidbacher, H.
The 350 km long Austrian Danube River is characterized by a natural sequence of constrained and alluvial sections. Its hydromorphological character has been changed severely by systematic channelization since the mid-19th century and hydropower plant construction since the 1950s. An ecological assessment of the Danube River, especially regarding the fish fauna, clearly shows the present deficits compared with the Leitbild ("high ecological status") according to the Water Framework Directive of the EU. Intensive human uses constrain implementing comprehensive rehabilitation programs to achieve a "good ecological status". Since approx. 80 % of the river length can be provisionally classified as "heavily modified water bodies" due to chains of impoundments, an adapted Leitbild approach for the development as well as the evaluation of mitigation measures has to be elaborated to achieve a "good ecological potential". The range of Leitbild-specific rehabilitation measures is generally restricted within impoundments, in particular in alluvial sections. The most favourable opportunities are present at the heads of reservoirs, where dynamic hydromorphological conditions remain. In constrained sections, rehabilitation measures mainly involve improving the riparian zones, small-scale widenings and mitigating the river continuum both at tributary confluences and in the main channel. In alluvial sections the main focus is on re-establishing lateral and longitudinal connectivity as well as fluvial dynamics. Against this background the present paper describes various types of rehabilitation measures deduced from historical references of alluvial Danube river sections.